Anti-phospholipid antibodies were measured in 9 patients with active thrombotic thrombocytopenic purpura (TTP). 8 patients had primary TTP and one had TTP secondary to systemic lupus erythematosus (SLE). No patient showed circulating 'lupus' anticoagulants or false positive tests for syphilis. A solid phase immuno-assay for anti-cardiolipin antibodies (ACA) gave negative results in the patient with secondary TTP as well as in all but one case with primary TTP. ACA of IgG class were not found in any TTP patient while they were present in 10 out of 18 patients suffering from thrombocytopenia with active SLE. These data indicate that anti-phospholipid antibodies do not have a role in the development of thrombosis and thrombocytopenia with TTP.