Ninety-two families whose probands had leukemia or lymphoma and 69 control families have been studied with an intensive interview technique. Twenty-one multiple family instances of leukemia and lymphoma have been recorded, and an impressive aggregate of neoplastic involvement was observed. No difference was detected in the exposure to various animal and other environmental agents by the methods employed between the leukemialymphoma group and the control group. The occurrence of leukemia and lymphoma was 2.5 times more frequent in the family members of the leukemia-lymphoma proband group than in the control group. These data suggest that hereditary factors are important in the etiology of leukemia and lymphoma.