The lupus anticoagulant was found in the plasma of 31 of 60 patients with systemic lupus erythematosus and other connective tissue disorders (mixed connective tissue disease, systemic vasculitis, polyarteritis nodosa, primary sicca syndrome, discoid lupus, Behcet's syndrome, and systemic sclerosis). Strong associations were found with biological false positive seroreaction for syphilis and thrombocytopenia. The most striking association, however, was with the high prevalence of thrombosis. This tendency to thrombosis was independent of disease activity of systemic lupus erythematosus. The lupus anticoagulant appears to be a useful marker for a subset of patients with systemic lupus erythematosus at risk for the development of thromboembolic complications.