An otherwise healthy elderly man developed massive, life-threatening, sublingual bleeding associated with an idiopathic factor VIII inhibitor. The patient was treated wtih cyclophosphamide, steroids, factor VIII concentrates, and repeated plasmapheresis (including three times with NCI-IBM blood-cell separator). Rapid clinical and laboratory improvement occurred, with complete disappearance of the inhibitor. The patient has remained well, without evidence of an inhibitor, for 8 mo. The possible role of each of the therapeutic measures in the disappearance of the inhibitor and the possible pathogenetic mechanism of this disorder are discussed. A high mortality rate and a striking incidence of sublingual hematoma have been observed in cases in the literature.