To determine the clinical significance of antiphospholipid antibodies (aPL) in patients with immune thrombocytopenic purpura (ITP), anticardiolipin (aCL) (IgG and IgM) and lupus anticoagulant (LA) were sought at diagnosis in 215 ITP adults with platelets <50 x 10(9)/l. aPL (aCL and/or LA) were detected in 55 patients (26%): aCL alone in 39 (18%), aCL and LA in 15 (7%) and LA alone in one (0.5%). LA was significantly associated with high IgG-aCL levels (P = 0.001). Among age, sex, initial platelet count, bleeding score, acute or chronic ITP outcome, only younger age was significantly associated with LA-positivity (mean age 29 +/- 14 years vs. 45 +/- 20 years, P = 0.002). After a median follow-up of 31 months, 14/215 (7%) patients developed thrombosis (four arterial, 10 venous and/or pulmonary embolism); four of them (29%) had high aCL levels and LA. Multivariate analysis significantly associated thrombosis events only with age [hazard ratio (HR) = 1.6; 95% confidence interval (CI): 1.2-2.4], LA (HR: 9.9; 95% CI: 2.3-43.4) or high IgG-aCL level (HR: 7.5; 95% CI; 1.8-31.5). Although the thrombosis rate was low, the significant associations between thrombosis and LA or high aCL level suggest that aPL should be tested at ITP diagnosis.