A variety of coagulation abnormalities has been described in systemic lupus erythematosus (SLE). We report a unique SLE patient with an acquired factor XI deficiency. The low factor XI levels were not due to a classical coagulation inhibitor, but to a plasma factor (probably an immunoglobulin) that selectively bound factor XI. The factor XI deficiency improved after treatment with corticosteroids. Thus it is possible that in SLE immune complexes may bind clotting factors making them unavailable for use in the coagulation cascade.