Three generations of a family were affected by hematologic malignancies: the proband had hairy cell leukemia, his brother chronic lymphocytic leukemia, his nephew cutaneous and central nervous system lymphoma, and his father chronic granulocytic leukemia. There was no clinical evidence for immune deficiency in 17 unaffected family members. Most family members had normal serum immunoglobulins and peripheral B-cells, but some had decreased T cells. Lymphocyte responses to phytohemagglutinin and concanavalin A were markedly reduced, while pokeweed mitogen responses were essentially normal. Delayed hypersensitivity responses determined by response to a 23-antigen skin test panel were markedly diminished, as were viral antibody titers. The decreased immune function in these family members suggests a role for the immune system in the emergence of hairy cell leukemia and other lymphoid malignancies, though further follow-up will be needed to see if other family members develop malignancies.