A 24-member kindred is described in which four cases of acute myeloid leukemia (AML), and one case of myelodysplastic syndrome (MDS) occurred over three generations. The proband was diagnosed with AML at age 47; within 6 months, her sister, age 41, was diagnosed with MDS. The proband's father, grandfather and a paternal uncle all died of AML, preceded by a pre-leukemic phase. The five cases had several clinical features in common. In the two sisters and their paternal uncle, cytogenetic analyses of bone marrow cells revealed a common abnormality characterized by loss of the long arm of chromosome 5, del(5q). No constitutional cytogenetic abnormality was detected in mitogen-stimulated peripheral blood lymphocytes from the proband. In addition, there was no history of common environmental or occupational exposure in the family. The occurrence of AML and MDS in three generations of this family most likely resulted from a single gene defect with an autosomal dominant pattern of inheritance. The association with the somatic loss of 5q material in the leukemia cells of affected members suggests that a germline mutation of a leukemia suppressor gene located on 5q might be the primary event responsible for hereditary susceptibility to leukemia in this family.