A family is described in which two sisters developed acute myeloblastic leukemia (AML FAB M2) and erythroleukemia (FAB M6) at ages 60 and 53, 15 years apart. Their father was diagnosed as having chronic erythremic myelosis (DiGuglielmo syndrome) 24 years earlier at age 56. Cytogenetic analysis of bone marrow blasts in the patient with AML M2 revealed a complex hyperdiploid karyotype with clonal abnormalities -3, -5, del(5)(q13q23), -8, +i(8)(q10), -11, add(11) (q23), add(12)(p13), -13, +3mar. No environmental risk factors could be identified. Hematologic and cytogenetic analysis of all living first degree relatives of the affected persons revealed hematological abnormalities, but normal constitutional karyotypes of peripheral blood lymphocytes and no induction of cytogenetic abnormalities in folate deficient medium.