JMML accounts for 1-2% of childhood leukemias each year; in the United States, an estimated 25-50 new cases are diagnosed each year, which also equates to about 3 cases per million children. There is no known environmental cause for JMML. Since about 10% of patients are diagnosed before 3 months of age, it is thought that JMML is a congenital condition in these infants

Controversy remains today whether this disorder is a subtype of acute myeloid leukemia or myelodysplastic syndromes; however, it is currently classified as a form of AML.

Myelophthisis can occur in the setting of chronic myeloproliferative disease (e.g. myelofibrosis), leukemia, lymphoma, and metastatic carcinoma or myeloma. It is common in people who have chronic idiopathic myelofibrosis. It has been linked to small-cell lung cancer, breast cancer or prostate cancer that metastasizes to the bone marrow.

Historically, the most common cause of displacement of healthy bone marrow was tuberculosis.

Currently, the most common cause is displacement of bone marrow by metastatic cancer (extramedullary hematopoiesis tends to be modest). Other causes include myeloproliferative disorders (especially late-stage or spent polycythemia vera), granulomatous diseases, and (lipid) storage diseases. Myelofibrosis can occur in all of these.

Factors that may contribute to decreased RBC production include a decreased quantity of functioning hematopoietic tissue, disordered metabolism related to the underlying disorder, and, in some cases, erythrophagocytosis.

There have been few individual epidemiological studies of CMML, due to the difficulty in the disease classification. CMML has an estimated incidence of less than 1 per 100,000 persons per year.

The median age of diagnosis is 65–75. CMML has a propensity for males rather than females, at a ratio of 1.5–3:1.

Some cases of myelophthisis are thought to be related to the release of cytokines that simulate fibroblastic proliferation and fibrosis in the marrow.

CML accounts for 8% of all leukaemias in the UK, and around 680 people were diagnosed with the disease in 2011.

High amounts of ionizing radiation exposure can increase the risk of AML. Survivors of the atomic bombings of Hiroshima and Nagasaki had an increased rate of AML, as did radiologists exposed to high levels of X-rays prior to the adoption of modern radiation safety practices. People treated with ionizing radiation after treatment for prostate cancer, non-Hodgkin lymphoma, lung cancer, and breast cancer have the highest chance of acquiring AML, but this increased risk returns to the background risk observed in the general population after 12 years.

Myelofibrosis, also known as osteomyelofibrosis, is a relatively rare bone marrow cancer. It is currently classified as a myeloproliferative neoplasm, in which the proliferation of an abnormal clone of hematopoietic stem cells in the bone marrow and other sites results in fibrosis, or the replacement of the marrow with scar tissue.

The term "myelofibrosis" alone usually refers to primary myelofibrosis (PMF), also known as chronic idiopathic myelofibrosis (cIMF); the terms idiopathic and primary mean that in these cases the disease is of unknown or spontaneous origin. This is in contrast with myelofibrosis that develops secondary to polycythemia vera or essential thrombocythaemia. Myelofibrosis is a form of myeloid metaplasia, which refers to a change in cell type in the blood-forming tissue of the bone marrow, and often the two terms are used synonymously. The terms agnogenic myeloid metaplasia and myelofibrosis with myeloid metaplasia (MMM) are also used to refer to primary myelofibrosis.

Exposure to anticancer chemotherapy, in particular alkylating agents, can increase the risk of subsequently developing AML. The risk is highest about three to five years after chemotherapy. Other chemotherapy agents, specifically epipodophyllotoxins and anthracyclines, have also been associated with treatment-related leukemias, which are often associated with specific chromosomal abnormalities in the leukemic cells.

Occupational chemical exposure to benzene and other aromatic organic solvents is controversial as a cause of AML. Benzene and many of its derivatives are known to be carcinogenic "in vitro". While some studies have suggested a link between occupational exposure to benzene and increased risk of AML, others have suggested the attributable risk, if any, is slight.

Leukemia is rarely associated with pregnancy, affecting only about 1 in 10,000 pregnant women. How it is handled depends primarily on the type of leukemia. Nearly all leukemias appearing in pregnant women are acute leukemias. Acute leukemias normally require prompt, aggressive treatment, despite significant risks of pregnancy loss and birth defects, especially if chemotherapy is given during the developmentally sensitive first trimester. Chronic myelogenous leukemia can be treated with relative safety at any time during pregnancy with Interferon-alpha hormones. Treatment for chronic lymphocytic leukemias, which are rare in pregnant women, can often be postponed until after the end of the pregnancy.

The primary sign of myelofibrosis is reactive bone marrow fibrosis, but it is often accompanied by:

- Abdominal fullness related to an enlarged spleen (splenomegaly).

- Bone pain

- Bruising and easy bleeding due to inadequate numbers of platelets

- Cachexia (loss of appetite, weight loss, and fatigue)

- Enlargement of both the liver and spleen

- Fatigue

- Gout and high uric acid levels

- Increased susceptibility to infection, such as pneumonia

- Pallor and shortness of breath due to anemia

- In rarer cases, a raised red blood cell volume

- Cutaneous myelofibrosis is a rare skin condition characterized by dermal and subcutaneous nodules.

This is a rare disease, with less than 100 cases reported. Of these cases, an equal male:female ratio was observed,

with cases typically seen in older adults.

Median survival is about 9 months.

Autologous stem cell transplantation has been used in treatment.

The Düsseldorf score stratifies cases using four categories, giving one point for each; bone marrow blasts ≥5%, LDH >200U/L, haemoglobin ≤9g/dL and a platelet count ≤100,000/uL. A score of 0 indicates a low risk group' 1-2 indicates an intermediate risk group and 3-4 indicates a high risk group. The cumulative 2 year survival of scores 0, 1-2 and 3-4 is 91%, 52% and 9%; and risk of AML transformation is 0%, 19% and 54% respectively.

Although not yet formally incorporated in the generally accepted classification systems, molecular profiling of myelodysplastic syndrome genomes has increased the understanding of prognostic molecular factors for this disease. For example, in low-risk MDS, "IDH1" and "IDH2" mutations are associated with significantly worsened survival.

Some people have a history of exposure to chemotherapy (especially alkylating agents such as melphalan, cyclophosphamide, busulfan, and chlorambucil) or radiation (therapeutic or accidental), or both (e.g., at the time of stem cell transplantation for another disease). Workers in some industries with heavy exposure to hydrocarbons such as the petroleum industry have a slightly higher risk of contracting the disease than the general population. Xylene and benzene exposure has been associated with myelodysplasia. Vietnam veterans exposed to Agent Orange are at risk of developing MDS. A link may exist between the development of MDS "in atomic-bomb survivors 40 to 60 years after radiation exposure" (in this case, referring to people who were in close proximity to the dropping of the atomic bomb in Hiroshima and Nagasaki during World War II).

Children with Down syndrome are susceptible to MDS, and a family history may indicate a hereditary form of sideroblastic anemia or Fanconi anemia.

Prognosis refers to how well a patient is expected to respond to treatment based on their individual characteristics at time of diagnosis. In JMML, three characteristic areas have been identified as significant in the prognosis of patients:

Without treatment, the survival [5 years?] of children with JMML is approximately 5%. Only Hematopoietic Stem Cell Transplantation (HSCT), commonly referred to as a bone marrow or (umbilical) cord blood transplant, has been shown to be successful in curing a child of JMML. With HSCT, recent research studies have found the survival rate to be approximately 50%. Relapse is a significant risk after HSCT for children with JMML. It is the greatest cause of death in JMML children who have had stem cell transplants. Relapse rate has been recorded as high as 50%. Many children have been brought into remission after a second stem cell transplant.

The exact cause of most cases of childhood leukemia is not known. Most children with leukemia do not have any known risk factors. The immune system plays an important role in protecting the body's immune system. An alteration or defect in the immune system may increase the risk for developing cancer. The immune system can be damaged by different factors, such as exposure to different viruses, environmental factors, chemical factors and other various infections.

There also appears to be some evidence linking childhood leukemia to x-ray exposure. In a 2010 study by the University of California, Berkeley’s School of Public Health, researchers found that children with acute lymphoid leukemia (ALL) had almost twice the chance of having been exposed to three or more X-rays compared with children who did not have leukemia.

Most cases of SPB progress to multiple myeloma within 2–4 years of diagnosis, but the overall median survival for SPB is 7–12 years. 30–50% of extramedullary plasmacytoma cases progress to multiple myeloma with a median time of 1.5–2.5 years. 15–45% of SPB and 50–65% of extramedullary plasmacytoma are disease free after 10 years.

The majority (90%) of cases have not had detectable cytogenetic abnormalities. Most importantly, the Philadelphia chromosome and other BCR/ABL fusion genes are not detected.

Some people have a genetic predisposition towards developing leukemia. This predisposition is demonstrated by family histories and twin studies. The affected people may have a single gene or multiple genes in common. In some cases, families tend to develop the same kinds of leukemia as other members; in other families, affected people may develop different forms of leukemia or related blood cancers.

In addition to these genetic issues, people with chromosomal abnormalities or certain other genetic conditions have a greater risk of leukemia. For example, people with Down syndrome have a significantly increased risk of developing forms of acute leukemia (especially acute myeloid leukemia), and Fanconi anemia is a risk factor for developing acute myeloid leukemia. Mutation in SPRED1 gene has been associated with a predisposition to childhood leukemia.

Chronic myelogenous leukemia is associated with a genetic abnormality called the Philadelphia translocation; 95% of people with CML carry the Philadelphia mutation, although this is not exclusive to CML and can be observed in people with other types of leukemia.

Plasmacytomas are a rare form of cancer. SPB is the most common form of the disease and accounts for 3-5% of all plasma cell malignancies. The median age at diagnosis for all plasmacytomas is 55. Both SPB and extramedullary plasmacytoma are more prevalent in males; with a 2:1 male to female ratio for SPB and a 3:1 ratio for extramedullary plasmacytoma.

Refractory cytopenia of childhood (RCC) is a subgroup of myelodysplastic syndrome (MDS), having been added to the World Health Organization classification in 2008. Before then, RCC cases were classified as childhood aplastic anemia. RCC is the most common form of MDS in children and adolescents, accounting for approximately half of all MDS cases.

Complete remission and long-term survival are more common in children than adults.

Prognosis depends upon the cause. One third of cases is associated with a t(1;22)(p13;q13) mutation in children. These cases carry a poor prognosis.

Another third of cases is found in Down syndrome. These cases have a reasonably fair prognosis.

The last third of cases may be heterogeneous, and carry a poor prognosis.

Symptoms result from underproduction of red blood cells (weakness, pallor, failure to thrive, pica), white blood cells (recurrent or overwhelming infection), and/or platelets (bleeding).

Bone marrow transplant is the only known curative treatment.