A new solid-phase radioimmunoassay for the detection of anticardiolipin antibodies is 200-400 times more sensitive than the precipitation method used in the Venereal Disease Reference Laboratory test. 61% of serum samples from patients with systemic lupus erythematosus (SLE) had high levels of anticardiolipin antibodies of at least one immunoglobulin class. There were strong correlations between raised anticardiolipin levels and the lupus anticoagulant, venous and arterial thrombosis, and thrombocytopenia, but no correlation with anti-DNA antibody levels. Of the 15 patients with the highest anticardiolipin titres, 6 had a history of venous thrombosis, 5 cerebral thrombosis, 5 thrombocytopenia, and 2 each pulmonary hypertension and multiple abortions. This simple immunoassay appears to have predictive value for thrombosis in SLE and related disorders.