The lupus anticoagulant was found in six of 41 unselected patients with systemic lupus erythematosus (14%). Three of these six patients had episodes of thrombosis. Thrombosis occurred in only one patient in the remainder of the series without the lupus anticoagulant. The lupus anticoagulant should be considered as one of the criteria for the diagnosis of systemic lupus erythematosus, and it may be a useful marker for those patients at risk from thromboembolism. It should be looked for in young adults with thrombotic episodes.