Acute Lymphocytic Leukemia is the most frequent cancer in childhood in developed countries with a peak of incidence between 1 and 4 years and a male predominance. Its incidence increases with economic development. Early exposure to infections seems to have a protective effect. The incidence of acute non-Lymphocytic Leukemia or acute myeloid leukemia seems less frequent in black population. Chronic myeloid leukemia is rare in children whilst chronic lymphocytic leukemia is not a disease of childhood.

NHL is the most frequent tumor of children in African units. This is due to high frequency of Burkitt lymphoma in sub-Saharan area of Africa. In North Africa this tumor is less frequent. Male predominance is reported in most series.

Hodgkin disease seems to occur at an earlier age in African children with also higher frequency of mixed cellularity subtype. Infection with the Epstein-Barr virus is considered as an important contributor to lymphomagenesis. Its genoma is also often found in the tumor tissue.

These tumors are the most frequent solid tumors in children in developed countries. These tumors seem to be less frequent in Africa. The diagnostic difficulties probably contribute to the apparent rarity of these tumors in this setting.

These tumors are rare in Africa. They represent 5–15 % in the European populations and North American. In most African countries there are seldom diagnosed.

These tumors seem of high prevalence in most African units of pediatric oncology representing the second or third most frequent tumor. Most patients are diagnosed at late stages, present with advanced disease and have consequently a bad prognosis. It is a tumor easy to diagnose and the examination of the eye should be part of each visit to the clinic.

The incidence of these tumors seems stable around the world. In Africa it is also most often diagnosed late, children presenting with huge abdomen and lung metastasis.

Liver tumors are more frequently reported in China, Japan, and Fiji Islands, where they exceed 3 per million inhabitants, while this incidence is 1–2 per million inhabitants in other regions of the world. In Africa there might be missed opportunities to diagnose and treat the disease.

Incidence of osteosarcoma is low in Asia (1–2 per million), intermediate (2–3.5 per million inhabitants) in North America and Europe, and high (more than 3.5 per million inhabitants) in the black American population and Brazil. In Africa, this tumor seems rare but the real incidence is not known.

This tumor is rare in children in Asia and Africa, not diagnosed or reported.

The incidence of these tumors is less than 5 per million in Asia. This incidence is around 8 per million inhabitants in Africa and Europe.

The incidence of germ cell tumors is higher in Japan and New Zealand. The peak incidence is bimodal, between 0 and 4 years with a male predominance and between 10 and 14 years with a female predominance.

These tumors are rare in children. A surge in the number of the tumors occurred in the population of Belarus after the accident of Chernobyl nuclear plant with a high female predominance.

These tumors are more common in North Africa and Asia (outside Japan) with predominance in adolescents and a male. Epstein-Barr virus is considered as initiating factor in the occurrence of this cancer.

Radiation is an established risk factor for cancer development. Excessive exposure in utero to X-ray or in childhood increases the risk of developing cancer. Radiation therapy may also be the origin of secondary cancers, especially acute leukemias, cancers of the thyroid, brain tumors, bone tumors, and breast cancers.

Secondary cancers are reported following treatment with the alkylating agents, epipodophyllotoxins, nitrosoureas, and anthracyclines. The risk is higher when chemotherapy is associated to radiation therapy. Secondary cancers usually occur 5 years or later after treatment.

Exposure to electromagnetic fields from high voltages, electrical appliances, or radio frequency transmission lines has been suspected as carcinogenesis factor. This has not been yet confirmed by controlled studies.

Extended exposure to pesticides increases the risk of leukemia, lymphoma, central nervous system tumors, neuroblastoma, and Wilms tumor. Exposure to solvents increases also the risk of leukemia. Maternal occupational exposure to coal, oil, dyes, or pigments is associated to an increase of the risk of occurrence of hepatoblastoma. Hodgkin disease and CNS tumors are also reported to occur with higher frequency in case of exposure to benzene and hydrocarbons. It was reported that the agricultural child labor in Africa increases the risk of occurrence of Ewing sarcoma which has not been demonstrated.

Several studies associated parental smoking with an increased risk of occurrence of non-Hodgkin, lymphoma, central nervous system tumors, neuroblastoma, and hepatoblastoma.

Maternal alcoholism during pregnancy increases the risk of neuroblastoma. Preconceptual paternal alcoholism also appears to increase the risk of tumors of the central nervous system.

A high birth weight seems to be a predisposing factor to acute lymphoblastic leukemia and Wilms tumor while low birth weight predisposes to hepatoblastoma.

Studies have shown a protective effect of breast-feeding children against cancer and in particular in case of maternal consumption of vegetable protein in the year before pregnancy. The administration of folate and iron during pregnancy seems also to have a protective effect.

Hair dye used during pregnancy appears to increase the risk of childhood brain tumors.

There is an increased risk for children born after in vitro fertilization to develop childhood cancer.

HIV infection increases the risk of developing chidlhood cancer and the occurrence of B-cell lymphoma and leiomyosarcoma. In this context, Kaposi sarcoma is more common and occurs after HHV8 virus coinfection. The EBV is associated in most cases of Burkitt Lymphoma in sub-Saharan Africa. It is also associated with Hodgkin lymphoma and carcinoma of the nasopharynx in North Africa and Southeast Asia. Hepatitis B and C are associated with hepatocellular carcinoma especially in older children and children not immunized. On the other hand, nursery attendance , chicken pox, poliomyelitis, and rubella appear to have a protective effect due to an increase in immunity.