The aim of this study was to determine the prevalence of platelet autoantibodies (PAA) in patients with systemic lupus erythematosus (SLE) and its correlation with clinical and other laboratory manifestations of the disease, as well as to evaluate the influence of platelet count and disease activity on the result of the test for PAA. Ninety SLE patients, 29 with thrombocytopenia, were evaluated. The presence of PAA was determined using the direct and indirect platelet suspension immunofluorescence test. A total of 166 PAA determinations were performed in the 90 patients upon entry into the study. Fifty-six of the 90 patients (62%) with SLE were positive for PAA. There were no statistically significant correlations between the presence of PAA and the different disease manifestations except for thrombocytopenia (P = 0.0005). The presence of PAA in the same patient was significantly associated with current thrombocytopenia and disease activity. It was concluded that the prevalence of PAA in SLE patients is high. All SLE patients with thrombocytopenia had PAA, although some patients with PAA had a normal platelet count. The presence of PAA in SLE patients is not synonymous with thrombocytopenia but should instead be regarded as one of the multiple serological abnormalities which may develop in these patients.