At least 22 members of a large kindred have a bleeding tendency resulting from an autosomal dominant disorder of platelet production and function. Phenotypic manifestations include mild to moderate thrombocytopenia, bleeding time prolongation, and abnormal platelet aggregation. Platelet survival time is normal. The platelet disorder in this family appears to differ from known hereditary thrombocytopenic or thrombocytopathic syndromes and may represent a new genetic disease. Six family members reportedly developed hematologic neoplasms: acute monocytic leukemia 9 years after treatment for congenital neuroblastoma; lymphosarcoma at age 10 years; myeloid leukemia at age 23 years; acute myelocytic leukemia at age 62 years; leukemia of unknown type at age 48 years; and lymphocytic lymphoma at age 52 years.