A patient with thrombotic thrombocytopenic purpura was treated with 8 U of fresh frozen plasma, resulting in a rapid improvement in her condition and subsequently followed by a prolonged clinical remission. The transmission and scanning electron microscopic examinations of the platelets performed before treatment demonstrated marked anomalies of the platelet morphology, namely, an increased number of abnormally elongated pseudopods with terminal knobs, a decreased number of platelet granules and excess of glycogen and marked anisocytosis, These anomalies remained unchanged when reexamined 2 months after the achievement of the clinical remission. The question whether these platelet abnormalities are an integral part of thrombotic thrombocytopenic purpura and play a role in its pathogenesis, is raised.