The most common symptoms in children are easy bruising, pale skin, fever, and an enlarged spleen or liver.

Damage to the bone marrow, by way of displacing the normal bone marrow cells with higher numbers of immature white blood cells, results in a lack of blood platelets, which are important in the blood clotting process. This means people with leukemia may easily become bruised, bleed excessively, or develop pinprick bleeds (petechiae).

White blood cells, which are involved in fighting pathogens, may be suppressed or dysfunctional. This could cause the patient's immune system to be unable to fight off a simple infection or to start attacking other body cells. Because leukemia prevents the immune system from working normally, some patients experience frequent infection, ranging from infected tonsils, sores in the mouth, or diarrhea to life-threatening pneumonia or opportunistic infections.

Finally, the red blood cell deficiency leads to anemia, which may cause dyspnea and .

Some patients experience other symptoms, such as feeling sick, having fevers, chills, night sweats, feeling fatigued and other flu-like symptoms. Some patients experience nausea or a feeling of fullness due to an enlarged liver and spleen; this can result in unintentional weight loss. Blasts affected by the disease may come together and become swollen in the liver or in the lymph nodes causing pain and leading to nausea.

If the leukemic cells invade the central nervous system, then neurological symptoms (notably headaches) can occur. Uncommon neurological symptoms like migraines, seizures, or coma can occur as a result of brain stem pressure. All symptoms associated with leukemia can be attributed to other diseases. Consequently, leukemia is always diagnosed through medical tests.

The word "leukemia", which means 'white blood', is derived from the characteristic high white blood cell count that presents in most afflicted patients before treatment. The high number of white blood cells are apparent when a blood sample is viewed under a microscope, with the extra white blood cells frequently being immature or dysfunctional. The excessive number of cells can also interfere with the level of other cells, causing further harmful imbalance in the blood count.

Some leukemia patients do not have high white blood cell counts visible during a regular blood count. This less-common condition is called "aleukemia". The bone marrow still contains cancerous white blood cells which disrupt the normal production of blood cells, but they remain in the marrow instead of entering the bloodstream, where they would be visible in a blood test. For an aleukemic patient, the white blood cell counts in the bloodstream can be normal or low. Aleukemia can occur in any of the four major types of leukemia, and is particularly common in hairy cell leukemia.

The way CML presents depends on the stage of the disease at diagnosis as it has been known to skip stages in some cases.

Most patients (~90%) are diagnosed during the chronic stage which is most often asymptomatic. In these cases it may be diagnosed incidentally with an elevated white blood cell count on a routine laboratory test. It can also present with symptoms indicative of hepatosplenomegaly and the resulting upper quadrant pain this causes. The enlarged spleen may put pressure on the stomach causing a loss of appetite and resulting weight loss. It may also present with mild fever and night sweats due to an elevated basal level of metabolism.

Some (<10%) are diagnosed during the accelerated stage which most often presents bleeding, petechiae and ecchymosis. In these patients fevers are most commonly the result of opportunistic infections.

Some patients are initially diagnosed in the blast phase in which the symptoms are most likely fever, bone pain and an increase in bone marrow fibrosis.

Clinically and pathologically, leukemia is subdivided into a variety of large groups. The first division is between its "acute" and "chronic" forms:

- Acute leukemia is characterized by a rapid increase in the number of immature blood cells. The crowding that results from such cells makes the bone marrow unable to produce healthy blood cells. Immediate treatment is required in acute leukemia because of the rapid progression and accumulation of the malignant cells, which then spill over into the bloodstream and spread to other organs of the body. Acute forms of leukemia are the most common forms of leukemia in children.

- Chronic leukemia is characterized by the excessive buildup of relatively mature, but still abnormal, white blood cells. Typically taking months or years to progress, the cells are produced at a much higher rate than normal, resulting in many abnormal white blood cells. Whereas acute leukemia must be treated immediately, chronic forms are sometimes monitored for some time before treatment to ensure maximum effectiveness of therapy. Chronic leukemia mostly occurs in older people, but can occur in any age group.

Additionally, the diseases are subdivided according to which kind of blood cell is affected. This divides leukemias into lymphoblastic or "lymphocytic leukemias" and myeloid or "myelogenous leukemias":

- In lymphoblastic or lymphocytic leukemias, the cancerous change takes place in a type of marrow cell that normally goes on to form lymphocytes, which are infection-fighting immune system cells. Most lymphocytic leukemias involve a specific subtype of lymphocyte, the B cell.

- In myeloid or myelogenous leukemias, the cancerous change takes place in a type of marrow cell that normally goes on to form red blood cells, some other types of white cells, and platelets.

Combining these two classifications provides a total of four main categories. Within each of these main categories, there are typically several subcategories. Finally, some rarer types are usually considered to be outside of this classification scheme.

Prognosis is very poor once chronic myelogenous leukemia reaches the accelerated phase; it behaves similarly to acute myeloid leukemia.

Acute leukemia or acute leukaemia is a family of serious medical conditions relating to an original diagnosis of leukemia. In most cases, these can be classified according to the lineage, myeloid or lymphoid, of the malignant cells that grow uncontrolled, but some are mixed and for those such an assignment is not possible.

Forms of acute leukemia include:

- Acute myeloid leukemia

- Acute erythroid leukemia

- Acute lymphoblastic leukemia

- T-cell acute lymphoblastic leukemia

- Adult T-cell leukemia/lymphoma

- (Precursor)T-lymphoblastic leukemia/lymphoma

- "Blast crisis" of chronic myelogenous leukemia

Approximately 85% of patients with CML are in the chronic phase at the time of diagnosis. During this phase, patients are usually asymptomatic or have only mild symptoms of fatigue, left side pain, joint and/or hip pain, or abdominal fullness. The duration of chronic phase is variable and depends on how early the disease was diagnosed as well as the therapies used. In the absence of treatment, the disease progresses to an accelerated phase. Precise patient staging based on clinical markers and personal genomic profile will likely prove beneficial in the assessment of disease history with respect to progression risk.

Patients treated with imatinib, dasatinib, and nilotinib have shown meaningful rates of hematologic and cytogenetic response.

Chronic leukemia is an increase of abnormal white blood cells. It differs from acute leukemia, and is categorized as myelogenous or lymphocytic.

Chronic leukemia may refer to:

- Chronic myelogenous leukemia

- Chronic lymphocytic leukemia

- Hairy cell leukemia

A specific histochemical reaction, cyanide-resistant peroxidase, permits identification of leukemic blast cells with eosinophilic differentiation and diagnosis of acute eosinoblastic leukemia in some cases of AML with few identifiable eosinophils in blood or marrow.

Very rarely, chloroma can occur without a known pre-existing or concomitant diagnosis of acute leukemia, acute promyleocytic leukemia or MDS/MPS; this is known as primary chloroma. Diagnosis is particularly challenging in this situation (see below). In almost all reported cases of primary chloroma, acute leukemia has developed shortly afterward (median time to development of acute leukemia 7 months, range 1–25 months). Therefore, primary chloroma could be considered an initial manifestation of acute leukemia, rather than a localized process, and could be treated as such. Where disease development or markers indicate progresses to acute promyleocytic leukemia (AML3) treatment should be tailored to this form of disease.

The following symptoms are typical ones which lead to testing for JMML, though children with JMML may exhibit any combination of them: pallor, fever, infection, bleeding, cough, poor weight gain, a maculopapular rash (discolored but not raised, or small and raised but not containing pus), lymphadenopathy (enlarged lymph nodes), moderate hepatomegaly (enlarged liver), marked splenomegaly (enlarged spleen), leukocytosis (high white blood cell count in blood), absolute monocytosis (high monocyte count in blood), anemia (low red blood cell count in blood), and thrombocytopenia (low platelet count in blood). Most of these conditions are common, nonspecific signs and symptoms.

Children with JMML and neurofibromatosis 1 (NF1) (about 14% of children with JMML are also clinically diagnosed with NF1, though up to 30% carry the NF1 gene mutation) may also exhibit any of the following symptoms associated with NF1 (in general, only young children with NF1 are at an increased risk of developing JMML):

- 6 or more café-au-lait (flat, coffee-colored) spots on the skin

- 2 or more neurofibromas (pea-size bumps that are noncancerous tumors) on or under the skin

- Plexiform neurofibromas (larger areas on skin that appear swollen)

- Optic glioma (a tumor on the optic nerve that affects vision)

- Freckles under the arms or in the groin

- 2 or more Lisch nodules (tiny tan or brown-colored spots on the iris of the eye)

- Various bone deformations including bowing of the legs below the knee, scoliosis, or thinning of the shin bone

Noonan syndrome (NS) may predispose to the development of JMML or a myeloproliferative disorder (MPD) associated with NS (MPD/NS) which resembles JMML in the first weeks of life. However, MPD/NS may resolve without treatment. Children with JMML and Noonan's syndrome may also exhibit any of the following most-common symptoms associated with Noonan's syndrome:

- Congenital heart defects, in particular, pulmonic stenosis (a narrowing of the valve from the heart to the lungs)

- Undescended testicles in males

- Excess skin and low hair line on back of neck

- Widely set eyes

- Diamond-shaped eyebrows

- Ears that are low-set, backward-rotated, thick outer rim

- Deeply grooved philtrum (upper lip line)

- Learning delays

Myeloid leukemia is a type of leukemia affecting myeloid tissue.

Types include:

- Acute myeloid leukemia

- Chronic myelogenous leukemia

Most people are diagnosed without symptoms as the result of a routine blood test that shows a high white blood cell count. Less commonly, CLL may present with enlarged lymph nodes without a high white blood cell count or no evidence of the disease in the blood. This is referred to as small lymphocytic lymphoma. In some individuals, the disease comes to light only after the cancerous cells overwhelm the bone marrow resulting in anemia producing tiredness or weakness.

Most initial symptoms of leukemia are similar to symptoms for irregular bone-marrow function. Typically, most symptoms do not occur during the early stages of leukemia, and children may experience different symptoms. The following are symptoms of leukemia that lead doctors to look for different types of juvenile leukemia:

Monocytic leukemia is a type of myeloid leukemia characterized by a dominance of monocytes in the marrow. When the monocytic cells are predominantly monoblasts, it can be subclassified into acute monoblastic leukemia.

Monocytic leukemia is almost always broken down into "acute" and "chronic":

- acute monocytic leukemia

- chronic myelomonocytic leukemia

Historically, they have been most commonly divided by the stage of maturation at which the clonal (neoplastic) lymphoid population stopped maturing:

- Acute lymphoblastic leukemia

- Chronic lymphocytic leukemia

However, the influential WHO Classification (published in 2001) emphasized a greater emphasis on cell lineage. To this end, lymphoid leukemias can also be divided by the type of cells affected:

- B-cell leukemia

- T-cell leukemia

- NK-cell leukemia

The most common type of lymphoid leukemia is B-cell chronic lymphocytic leukemia.

Acute eosinophilic leukemia is treated as other subtypes of AML. Response to treatment is approximately the same as in other types of AML.

Chloromas may occur in virtually any organ or tissue. The most common areas of involvement are the skin (also known as "leukemia cutis") and the gums. Skin involvement typically appears as violaceous, raised, nontender plaques or nodules, which on biopsy are found to be infiltrated with myeloblasts

Other tissues which can be involved include lymph nodes, the small intestine, the mediastinum, the lung, epidural sites, the uterus, the ovaries, and the orbit of the eye. Symptoms of chloroma at these sites are related to their anatomic location; chloromas may also be asymptomatic and be discovered incidentally in the course of evaluation of a person with acute myeloid leukemia.

Central nervous system involvement, as described above, most often takes the form of "meningeal leukemia", or invasion of the subarachnoid space by leukemic cells. This condition is usually considered separately from chloroma, as it requires different treatment modalities. True chloromas (i.e. solid leukemic tumors) of the central nervous system are exceedingly rare, but has been described.

"B-cell leukemia" describes several different types of lymphoid leukemia which affect B cells.

Other types include (with ICD-O code):

- 9826/3 – Acute lymphoblastic leukemia, mature B-cell type

- 9833/3 – B-cell prolymphocytic leukemia

- 9940/3 – Hairy cell leukemia

Chronic lymphoid leukemia (CLL) is a type of cancer in which the bone marrow makes too many lymphocytes (a type of white blood cell). Early on there is typically no symptoms. Latter non-painful lymph nodes swelling, feeling tired, fever, or weight loss for no clear reason may occur. Enlargement of the spleen and anemia may also occur. It typically worsens gradually.

Risk factors include having a family history of the disease. Agent Orange and certain insecticides might also be a risk. CLL results in the build up of B cell lymphocytes in the bone marrow, lymph nodes, and blood. These cells do not function well and crowd out healthy blood cells. It is divided into two main types those with a mutated IGHV gene and those without. Diagnosis is typically based on blood tests finding high numbers of mature lymphocytes and smudge cells.

Management of early disease is generally with watchful waiting. Infections should more readily be treated with antibiotics. In those with significant symptoms chemotherapy or immunotherapy may be used. The medications fludarabine, cyclophosphamide, and rituximab are typically the initial treatment in those who are otherwise healthy.

CLL affected about 904,000 people globally in 2015 and resulted in 60,700 deaths. The disease most common occurs in people over the age of 50. Males are affected more often than females. It is much less common in people from Asia. Five-year survival following diagnosis is approximately 83% in the United States. It represents less than 1% of deaths from cancer.

Childhood leukemia is a type of leukemia, usually acute lymphocytic leukemia (ALL), and a type of childhood cancer. The cure rate of childhood leukemia is generally higher than adult leukemia, approaching 90%, although some side effects of treatment last into adulthood. The older aggressive treatments of cranial irradiation and anthracyclines (such as doxorubicin) caused increased risk of solid tumors, heart failure, growth retardation, and cognitive defects.

Leukemia is a hematological malignancy or a cancer of the blood. It develops in the bone marrow, the soft inner part of bones where new blood cells are made. When a child has leukemia, the bone marrow produces white blood cells that do not mature correctly. Normal healthy cells only reproduce when there is enough space for them. The body will regulate the production of cells by sending signals of when to stop production. When a child has leukemia, the cells do not respond to the signals telling them when to stop and when to produce cells, regardless of the available space.

In hairy cell leukemia, the "hairy cells" (malignant B lymphocytes) accumulate in the bone marrow, interfering with the production of normal white blood cells, red blood cells, and platelets. Consequently, patients may develop infections related to low white blood cell count, anemia and fatigue due to a lack of red blood cells, or easy bleeding due to a low platelet count. Leukemic cells may gather in the spleen and cause it to swell; this can have the side effect of making the person feel full even when he or she has not eaten much.

Hairy cell leukemia is commonly diagnosed after a routine blood count shows unexpectedly low numbers of one or more kinds of normal blood cells, or after unexplained bruises or recurrent infections in an otherwise apparently healthy patient.

Platelet function may be somewhat impaired in HCL patients, although this does not appear to have any significant practical effect. It may result in somewhat more mild bruises than would otherwise be expected for a given platelet count or a mildly increased bleeding time for a minor cut. It is likely the result of producing slightly abnormal platelets in the overstressed bone marrow tissue.

Patients with a high tumor burden may also have somewhat reduced levels of cholesterol, especially in patients with an enlarged spleen. Cholesterol levels return to more normal values with successful treatment of HCL.

Atypical chronic myeloid leukemia (aCML) is a type of leukemia. It is a heterogeneous disorder belonging to the group of myelodysplastic/myeloproliferative (MDS/MPN) syndromes.

In aCML many clinical features (splenomegaly, myeloid predominance in the bone marrow with some dysplastic features but without a differentiation block) and laboratory abnormalities (myeloid proliferation, low leukocyte alkaline phosphatase values) suggest the diagnosis of chronic myelogenous leukemia (CML). However the lack of the pathognomonic Philadelphia chromosome and of the resulting BCR-ABL1 fusion point to a different pathogenetic process. Since no specific recurrent genomic or karyotypic abnormalities have been identified in aCML, the molecular pathogenesis of this disease has remained elusive and the outcome dismal (median survival 37 months) with no improvement over the last 20 years. This sharply contrasts with the outcome for CML, for which the prognosis was dramatically improved by the development of imatinib as a specific inhibitor of the BCR-ABL protein and in particular for CML.

In 2012 "SETBP1" was identified as a novel oncogene in aCML; specific somatic mutations of this gene were discovered in people with aCML and related diseases. These mutations, which are identical to the ones present in SGS as germline mutations, impair the degradation of SETBP1 and therefore cause increased cellular levels of the protein.

Juvenile myelomonocytic leukemia (JMML) is a serious chronic leukemia (cancer of the blood) that affects children mostly aged 4 and younger. The name JMML now encompasses all diagnoses formerly referred to as juvenile chronic myeloid leukemia (JCML), chronic myelomonocytic leukemia of infancy, and infantile monosomy 7 syndrome. The average age of patients at diagnosis is 2 years old. The World Health Organization has included JMML in the category of myelodysplastic and myeloproliferative disorders.

People affected by T-cell prolymphocytic leukemia typically have systemic disease at presentation, including enlargement of the liver and spleen, widespread enlargement of the lymph nodes, and skin infiltrates.

Due to the systemic nature of this disease, leukemic cells can be found in peripheral blood, lymph nodes, bone marrow, spleen, liver, and skin. A high lymphocyte count (> 100 x 10/L) along with low amounts of red blood cells and platelets in the blood are common findings. HTLV-1 serologies are negative, and serum immunoglobins are within normal limits with no paraproteins present.