Childhood acute lymphoblastic leukemia (ALL) T and B precursor subtypes have been identified by standardised immunophenotyping in different geographic and ethnic settings. Comparison of the relative frequencies and estimated incidence rates of the major subtypes indicates very similar values, with the striking exception of black childhood populations in Africa in which there appears to be a significant and selective deficit in the incidence of the common (B-cell precursor) subset of ALL. There is suggestive evidence for a similar bias in ALL subtypes in South Africans of mixed ethnic origin and in Mapuche Indians from Chile. Several interpretations of these data are possible but the one favoured attributes these differences primarily to socio-economic factors and patterns of infection in infancy.