OBJECTIVE Several mechanisms in formation and destruction of platelets could be involved in thrombocytopenia associated with HIV infection (TP/HIV). This epidemiological study investigated the pathogenic role of immunoglobulins associated with platelets (IAP), circulant immunocomplexes (CIC) and anticardiolipin antibodies (ACA) in patients with TP/HIV: METHODS A total of 207 adults patients infected with HIV were studied. Patients were classified as thrombocytopenic (platelet count < 100,000/mm3, n = 68); borderline thrombocytopenic (platelet count from 100,000 to 150,000/mm3, n = 23) and non-thrombopenic (platelet count > 150,000/m3, n = 116). IAP were investigated by an immunofluorescence technique with flow cytometry reading and eluate technique. CIC were investigated by C1q fixation measured by nephelometry. IgG-ACA determination was made with a commercially available ELISA technique. RESULTS Prevalence of thrombocytopenia in the general cohort of seropositive patients was 16%. Fifteen per cent of these patients had severe TP. There were no significant differences in epidemiology or prognosis among patients with and without TP. Patients with TP/HIV had increase rates of IAP, CIC and ACA (78%, 42% and 89%). These parameters were also increased in a similar percentage of non-TP infected patients (73%, 52%, 94% respectively). No correlation was observed between platelet counts and values of these immunological phenomena. CONCLUSIONS TP/HIV is common, mild, with no prognostic significance and occurs in an heterogeneous patient population. Immune phenomena associated with a decreased platelet survival occur nonspecifically and with an uncertain pathogenic meaning.