Since November 1980 we have diagnosed 11 cases of severe autoimmune thrombocytopenic purpura in homosexual men; their mean platelet count (± SE) was 16 000 ± 3000/mm³. All patients have been sexually active with multiple partners and exposed to numerous viruses and drugs. During this period, we also have diagnosed 20 cases of classic autoimmune thrombocytopenic purpura in heterosexual persons, with a normal women to men ratio of 3:1. Eight of nine homosexual patients had elevated platelet IgG compared with normal values in eight of 10 homosexual control subjects having normal hemograms (p < 0.01). All responded moderately or completely to steroids. The three patients who had splenectomy had excellent responses. Four of five patients had a decreased helper/suppressor T cell ratio compared to healthy controls (p < 0.001). Circulating immune complexes and total gamma globulin levels were elevated and lymphocytes relatively decreased in homosexual patients compared with homosexual controls (p < 0.05). Thus, some sexually-active homosexual men seem to have an increased incidence of an immune regulation disorder directed against platelets.