Two instances of leukemia in siblings which occurred in two families are reported. One patient had microcephalia in addition to acute granulocytic leukemia. The parents were first cousins. Both mothers of the involved siblings had histories of early entry into Hiroshima City after the atomic bomb explosion and prior to their marriages. While a probable role of consanguineous marriage on the occurrence of familial leukemia is supported by previously reported studies, a leukemogenic effect of parental exposure to atomic bomb radiation has not been shown.