The records of 608 intravenous drug addicts and 124 hemophiliacs diagnosed in 1985 to be seropositive for HIV were reviewed to identify patients with thrombocytopenia. The records of 54 drug addicts (8.9%) and 14 hemophiliacs (11.2%) who had chronic thrombocytopenia were then followed through June 1989 to estimate the incidence of bleeding episodes. The records of 55 HIV-seronegative patients with idiopathic thrombocytopenic purpura (ITP), comparable for age, platelet count at diagnosis and duration of follow-up, were used for comparison. Patients with ITP were significantly different from those with HIV-associated thrombocytopenia in having a higher proportion of females (M/F ratio 13/42 v 53/15, p less than 0.001) and more intensive treatment (treated: 62% v 21%, p less than 0.001; splenectomized 37% v 1.5%, p less than 0.001). The incidence of major bleeding was not significantly different in drug addicts, hemophiliacs and ITP patients (0.98, 1.82 and 1.17/100 patients/year). Among drug addicts, bleeding was significantly more frequent in women than in men (3.4 v 0/100 patients/year, p less than 0.05). Our results indicate that the incidence of major bleeding in patients with HIV-associated thrombocytopenia is low and similar to that observed in more intensively treated ITP patients.