Fourteen male patients examined for a prolonged partial thromboplastin time were found to have the lupus anticoagulant. In contrast to previous reports, there was no increased incidence of false-positive results of serological tests for syphilis. In only two patients was systemic lupus erythematosus confirmed, although two additional patients had a positive result of a test for antinuclear antibody. Other clinical diagnoses included peripheral vascular disease, cardiac disease, pulmonary disease, and schizophrenia. Prothrombin times were distinctly abnormal in only two patients. Bleeding was rarely encountered in these patients, including ten who underwent surgical procedures or some type of hemostatic challenge. Thrombocytopenia was not associated with bleeding but was present in two patients who had thrombotic events.