Results of a study on the relative frequencies of tumors in American black and Nigerian children were compared with data from the Childhood Cancer Registries in Manchester, United Kingdom, and Kampala, Uganda. The American black child living in Washington, D.C. and the Caucasian child living in Manchester had similar high frequencies for leukemia and glioma, whereas the incidence of lymphoma and retinoblastoma was low. African children living in Nigeria or Uganda had the opposite frequency patterns. These differences in frequencies of tumors between two ethnologically related population groups, American black and Nigerian, suggested the influence of environmental factors in the etiology of these tumors, even though exposure to environmental carcinogens was short. The rarity of Ewing's sarcoma and testicular tumors in American black and Nigerian children suggested a genetic influence.