The average age of diagnosis is between 15 and 35 years. This is about 5 to 10 years older than men with other germ cell tumors of the testes. In most cases, they produce masses that are readily felt on testicular self-examination; however, in up to 11 percent of cases, there may be no mass able to be felt, or there may be testicular atrophy. Testicular pain is reported in up to one fifth of cases. Low back pain may occur after metastasis to the retroperitoneum.

Some cases of seminoma can present as a primary tumour outside the testis, most commonly in the mediastinum. In the ovary, the tumor is called a dysgerminoma, and in non-gonadal sites, particularly the central nervous system, it is called a germinoma.

Seminoma (also known as "pure seminoma" or "classical seminoma") is a germ cell tumor of the testicle or, more rarely, the mediastinum or other extra-gonadal locations. It is a malignant neoplasm and is one of the most treatable and curable cancers, with a survival rate above 95% if discovered in early stages.

Testicular seminoma originates in the germinal epithelium of the seminiferous tubules. About half of germ cell tumors of the testicles are seminomas. Treatment usually requires removal of one testicle. However, fertility usually isn't affected. All other sexual functions will remain intact.

Despite their name, germ cell tumors occur both within and outside the ovary and testis.

- head

- inside the cranium — pineal and suprasellar locations are most commonly reported

- inside the mouth — a fairly common location for teratoma

- neck

- mediastinum — account for 1% to 5% of all germ cell neoplasms

- pelvis, particularly sacrococcygeal teratoma

- ovary

- testis

In females, germ cell tumors account for 30% of ovarian tumors, but only 1 to 3% of ovarian cancers in North America. In younger women germ cell tumors are more common, thus in patients under the age of 21, 60% of ovarian tumors are of the germ cell type, and up to one-third are malignant. In males, germ cell tumors of the testis occur typically after puberty and are malignant (testicular cancer). In neonates, infants, and children younger than 4 years, the majority of germ cell tumors are sacrococcygeal teratomas.

Males with Klinefelter syndrome have a 50 times greater risk of germ cell tumors (GSTs). In these persons, GSTs usually contain nonseminomatous elements, present at an earlier age, and seldom are gonadal in location.

Choriocarcinoma is a malignant, trophoblastic cancer, usually of the placenta. It is characterized by "early hematogenous spread" to the lungs. It belongs to the malignant end of the spectrum in gestational trophoblastic disease (GTD). It is also classified as a germ cell tumor and may arise in the testis or ovary.

One of the first signs of testicular cancer is often a lump or swelling in the testes. The U.S. Preventive Services Task Force (USPSTF) recommends against routine screening for testicular cancer in asymptomatic adolescent and adults including routine testicular self-exams. However, the American Cancer Society suggests that some men should examine their testicles monthly, especially if they have a family history of cancer, and the American Urological Association recommends monthly testicular self-examinations for all young men.

Symptoms may also include one or more of the following:

- a lump in one testis which may or may not be painful

- sharp pain or a dull ache in the lower abdomen or scrotum

- a feeling often described as "heaviness" in the scrotum

- breast enlargement (gynecomastia) from hormonal effects of β-hCG

- low back pain (lumbago) due to the cancer spreading to the lymph nodes along the back

It is not very common for testicular cancer to spread to other organs, apart from the lungs. If it has, however, the following symptoms may be present:

- shortness of breath (dyspnea), cough or coughing up blood (hemoptysis) from metastatic spread to the lungs

- a lump in the neck due to metastases to the lymph nodes

Testicular cancer, cryptorchidism, hypospadias, and poor semen quality make up the syndrome known as testicular dysgenesis syndrome.

Mixed germ cell tumors occur in many forms. Among these, a common form is teratoma with endodermal sinus tumor.

Teratocarcinoma refers to a germ cell tumor that is a mixture of teratoma with embryonal carcinoma, or with choriocarcinoma, or with both. This kind of mixed germ cell tumor may be known simply as a teratoma with elements of embryonal carcinoma or choriocarcinoma, or simply by ignoring the teratoma component and referring only to its malignant component: embryonal carcinoma and/or choriocarcinoma. They can present in the anterior mediastinum.

Although testicular cancer can be derived from any cell type found in the testicles, more than 95% of testicular cancers are germ cell tumors (GCTs). Most of the remaining 5% are sex cord-gonadal stromal tumours derived from Leydig cells or Sertoli cells. Correct diagnosis is necessary to ensure the most effective and appropriate treatment. To some extent, this can be done via blood tests for tumor markers, but definitive diagnosis requires examination of the histology of a specimen by a pathologist.

Most pathologists use the World Health Organization classification system for testicular tumors:

- Germ cell tumors

- "Precursor lesions"

- Intratubular germ cell neoplasia

- Unclassified type (carcinoma in situ)

- Specified types

- "Tumors of one histologic type (pure forms)"

- Seminoma

- Variant - Seminoma with syncytiotrophoblastic cells

- Spermatocytic seminoma

- Variant - spermatocytic seminoma with sarcoma

- Embryonal carcinoma

- Yolk sac tumor

- Trophoblastic tumors

- Choriocarcinoma

- Variant - monophasic choriocarcinoma

- Placental site trophoblastic tumour

- Cystic trophoblastic tumor

- Teratoma

- Variant - Dermoid cyst

- Variant - Epidermoid cyst

- Variant - Monodermal teratoma (Carcinoid, Primitive neuroectodermal tumor (PNET), Nephroblastoma-like tumor, others.

- Variant - Teratomic with somatic-type malignancy

- "Tumours of more than one histologic type (mixed forms)"

- Embryonal carcinoma and teratoma

- Teratoma and seminoma

- Choriocarcinoma and teratoma.embryonal carcinoma

- Others

- Sex cord/Gonadal stromal tumors

- Leydig cell tumor

- Sertoli cell tumor

- Lipid rich variant

- Scleriosing variant

- Large cell calcifying variant

- Intratubular sertoli cell neoplasia in Peutz-Jeghers syndrome

- Granulosa cell tumor

- Adult type

- Juvenile type

- Thecoma Fibroma Group

- Thecoma

- Fibroma

- Sex cord/gonadal stromal tumor - incompletely differentiated

- Sex cord/gonadal stromal tumor - mixed types

- Mixed Germ Cell and Sex Cord/Gonadal Stromal Tumors

- Gonadoblastoma

- Germ cell-sex cord/gonadal stromal tumor, unclassified

- Miscellaneous tumours of the testis

- Carcinoid

- Tumors of ovarian epithelial types

- Serous tumor of borderline malignancy

- Serous carcinoma

- Well differentiated endometrioid tumor

- Mucinous cystadenoma

- Mucinous cystadenocarcinoma

- Brenner tumor

- Nephroblastoma

- Paraganglioma

- Haematopoietic tumors

- Tumours of collecting ducts and rete

- Adenoma

- Carcinoma

- Tumors of the paratesticular structures

- Adenomatoid tumor

- Malignant and Benign Mesothelioma

- Adenocarcinoma of the epididymis

- Papillary cystadenoma of the epididymis

- Melanotic neuroectodermal tumor

- Desmoplastic small round cell tumor

- Mesenchymal tumors of the spermatic cord and testicular adnexae

- Lipoma

- Liposarcoma

- Rhabdomyosarcoma

- Aggressive angiomyxoma

- Angiomyofibroblastoma-like tumor (see Myxoma)

- Fibromatosis

- Fibroma

- Solitary fibrous tumor

- Others

- Secondary tumors of the testis

These tumours are of the following types, characterized by their abnormal production of otherwise apparently normal cells or tissues.

Although each of the cell and tissue types normally occurs in just one sex (male or female), within a tumour they can occur in the opposite sex. Consequently, depending on the specific histology produced, these tumours can cause virilization in women and feminization in men.

Spermatocytic seminoma is a rare tumour, making up only one to two percent of all testicular germ cell tumours. Men presenting with this tumour are generally 50 to 60 years old, and its occurrence is rare in men under 30 years old. Most present with slow, painless testicular enlargement, which may involve both testes.

GCNIS is seen in the following settings:

- Almost all invasive germ cell tumours of the testis in adults

- Fifty percent of patients with GCNIS developed invasive germ cell tumours within five years of initial diagnosis.

- Five percent of contralateral testes in men with a history of prior testicular germ cell tumour.

- Less than five percent of cryptorchid testes.

- Less than one percent of patients with infertility.

Choriocarcinoma of the placenta during pregnancy is preceded by:

- hydatidiform mole (50% of cases)

- spontaneous abortion (20% of cases)

- ectopic pregnancy (2% of cases)

- normal term pregnancy (20–30% of cases)

- hyperemesis gravidarum

Rarely, choriocarcinoma occurs in primary locations other than the placenta; very rarely, it occurs in testicles. Although trophoblastic components are common components of mixed germ cell tumors, pure choriocarcinoma of the adult testis is rare. Pure choriocarcinoma of the testis represents the most aggressive pathologic variant of germ cell tumors in adults, characteristically with early hematogenous and lymphatic metastatic spread. Because of early spread and inherent resistance to anticancer drugs, patients have poor prognosis. Elements of choriocarcinoma in a mixed testicular tumor have no prognostic importance.

Choriocarcinomas can also occur in the ovaries.

Sex cord–gonadal stromal tumour (or sex cord–stromal tumour) is a group of tumors derived from the stromal component of the ovary and testis, which comprises the granulosa, thecal cells and fibrocytes. In contrast, the epithelial cells originate from the outer epithelial lining surrounding the gonad while the germ cell tumors arise from the precursor cells of the gametes, hence the name germ cell. In humans, this group accounts for 8% of ovarian cancers and under 5% of testicular cancers. Their diagnosis is histological: only a biopsy of the tumour can make an exact diagnosis. They are often suspected of being malignant prior to operation, being solid ovarian tumours that tend to occur most commonly in post menopausal women.

This group of tumours is significantly less common than testicular germ cell tumours in men, and slightly less common than ovarian germ cell tumours in women (see Ovarian cancer).

In the testis pure embryonal carcinoma is also uncommon, and accounts for approximately ten percent of testicular germ cell tumours. However, it is present as a component of almost ninety percent of mixed nonseminomatous germ cell tumours. The average age at diagnosis is 31 years, and typically presents as a testicular lump which may be painful. One fifth to two thirds of patients with tumours composed predominantly of embryonal carcinoma have metastases at diagnosis.

Not all germ cell tumors (GCTs) arise from "intratubular germ cell neoplasia". The following testicular GCTs do not arise from ITGCN:

- Spermatocytic seminoma

- Pediatric Yolk sac tumors (endodermal sinus tumour). This is currently an area of controversy as some authors dispute the absence of ITGCN in these cases.

- Teratoma (rare exceptions)

In the ovary, embryonal carcinoma is quite rare, amounting to approximately three percent of ovarian germ cell tumours. The median age at diagnosis is 15 years. Symptoms and signs are varied, and may include sexual precocity and abnormal (increased, reduced or absent) uterine bleeding.

There may be elevations in serum human chorionic gonadotropin (hCG) and alpha fetoprotein (AFP) levels but it would be in association with other tumors, (e.g. yolk sac tumor) because they themselves do not produce the serum markers. At surgery, there is extension of the tumour beyond the ovary in forty percent of cases. They are generally large, unilateral tumours, with a median diameter of 17 centimetres. Long-term survival has improved following the advent of chemotherapy. The gross and histologic features of this tumour are similar to that seen in the testis.

Trophoblastic neoplasms are neoplasms which derive from trophoblastic tissue.

Examples include:

- choriocarcinoma

- hydatidiform mole

They are exceptionally associated with hypercalcemia. On gross examination, dysgerminomas present with a smooth, bosselated (knobby) external surface, and is soft, fleshy and either cream-coloured, gray, pink or tan when cut. Microscopic examination typically reveals uniform cells that resemble primordial germ cells. Typically, the stroma contains lymphocytes and about 20% of patients have sarcoid-like granulomas.

Metastases are most often present in the lymph nodes.

Spermatocytic seminoma is not considered a subtype of seminoma and, unlike seminoma and most other germ cell tumours, it does not arise from "intratubular germ cell neoplasia". It has not been described as arising in locations outside the testis, and does not occur in association with other germ cell tumours.

The new WHO classification, due to be published in 2016, will rename the condition spermatocytic tumour to avoid confusion with classical seminoma as the biology and treatment are different.

Estrogens are produced by "functioning" tumours, and the clinical presentation depends on the patient's age and sex.

- Female

- If the patient is postmenopausal, she usually presents with abnormal uterine bleeding, and in some cases hemoperitoneum.

- If the patient is of reproductive age, she would present with menometrorrhagia. However, in some cases she may stop ovulating altogether.

- If the patient has not undergone puberty, early onset of puberty may be seen.

- these tumors tend to have late recurrencies ( even after 30 years )

Teratomas maybe found in babies, children, and adults. Teratomas of embryonal origin are most often found in babies at birth, in young children, and, since the advent of ultrasound imaging, in fetuses.

The most commonly diagnosed fetal teratomas are sacrococcygeal teratoma (Altman types I, II, and III) and cervical (neck) teratoma. Because these teratomas project from the fetal body into the surrounding amniotic fluid, they can be seen during routine prenatal ultrasound exams. Teratomas within the fetal body are less easily seen with ultrasound; for these, MRI of the pregnant uterus is more informative.

Teratomas are not dangerous for the fetus unless there is either a mass effect or a large amount of blood flow through the tumor (known as "vascular steal"). The mass effect frequently consists of obstruction of normal passage of fluids from surrounding organs. The vascular steal can place a strain on the growing heart of the fetus, even resulting in heart failure, and thus must be monitored by fetal echocardiography.

Teratomas can cause an autoimmune illness called Anti N-methyl-D-aspartate (NMDA) Receptor Encephalitis. After surgery, there is a risk of regrowth in place, or in nearby organs.

A dysgerminoma is a type of germ cell tumor; it usually is malignant and usually occurs in the ovary.

A tumor of the identical histology but not occurring in the ovary may be described by an alternate name: seminoma in the testis or germinoma in the central nervous system or other parts of the body.

Dysgerminoma accounts for less than 1% of ovarian tumors overall. Dysgerminoma usually occurs in adolescence and early adult life; about 5% occur in pre-pubertal children. Dysgerminoma is extremely rare after age 50. Dysgerminoma occurs in both ovaries in 10% of patients and, in a further 10%, there is microscopic tumor in the other ovary.

Abnormal gonads (due to gonadal dysgenesis and androgen insensitivity syndrome) have a high risk of developing a dysgerminoma. Most dysgerminomas are associated with elevated serum lactic dehydrogenase (LDH), which is sometimes used as a tumor marker.

Granulosa cell tumours (or granulosa-theca cell tumours or folliculoma) are tumours that arise from granulosa cells. These tumours are part of the sex cord-gonadal stromal tumour or non-epithelial group of tumours. Although granulosa cells normally occur only in the ovary, granulosa cell tumours occur in both ovaries and testicles (see Ovarian cancer and Testicular cancer). These tumours should be considered malignant and treated in the same way as other malignant tumours of ovary. The ovarian disease has two forms, juvenile and adult, both characterized by indolent growth, and therefore has high recovery rates.

The staging system for these tumours is the same as for epithelial tumours and most present as stage I. The peak age at which they occur is 50–55 years, but they may occur at any age.

Juvenile granulosa cell tumour is a similar but distinct rare tumour. It too occurs in both the ovary and testis. In the testis it is extremely rare, and has not been reported to be malignant. Although this tumour usually occurs in children (hence its name), it has been reported in adults.

Dysgerminoma is the most common type of malignant germ cell ovarian cancer. Dysgerminoma usually occurs in adolescence and early adult life; about 5% occur in pre-pubertal children. Dysgerminoma is extremely rare after age 50. Dysgerminoma occurs in both ovaries in 10% of patients and, in a further 10%, there is microscopic tumor in the other ovary.

Seminoma is the second most common testicular cancer; the most common is mixed, which may contain seminoma.

Abnormal gonads (due to gonadal dysgenesis and androgen insensitivity syndrome) have a high risk of developing a dysgerminoma. Most dysgerminomas are associated with elevated serum lactic dehydrogenase (LDH), which is sometimes used as a tumor marker.

Metastases are most often present in the lymph nodes.

An older system of classification structures polyorchidism into similar types, but with no subdivision between connected and disconnected testicles:

- Type 1: The testicle lacks an epididymis and vas deferens and has no connection to the other testicles.

- Type 2: The supernumerary testicle shares the epididymis and the vas deferens of the other testicles.

- Type 3: The supernumerary testicle has its own epididymis and shares a vas deferens.

- Type 4: Complete duplication of the testicle, epididymis and vas deferens.