The total incidence of childhood cancer varies rather little between different regions of the world, with cumulative risk to age 15 nearly always in the range 1.0-2.5 per thousand. Acute lymphoblastic leukaemia, especially in early childhood, is most common in populations of high socio-economic status and is the most frequent childhood cancer in all industrialised countries. The risk of Burkitt's lymphoma is highest in tropical Africa and Papua New Guinea; it is strongly associated with Epstein-Barr virus infection and intense immune stimulation by malaria. Other lymphomas are also relatively common in developing countries. Non-heritable retinoblastoma has a higher incidence among less affluent populations, suggesting an association with poor living conditions and maybe an infectious aetiology. In contrast, the incidence of Wilms' tumour and Ewing's sarcoma varies largely on ethnic lines, indicating a strong role for genetic predisposition. Much of the variation in recorded incidence of brain tumours and neuroblastoma may be due to varying levels of case ascertainment. Recently the incidence of childhood Kaposi's sarcoma has risen substantially in parts of Africa severely affected by the AIDS epidemic.