A therapeutic trial of prostacyclin (PGI), was done in a patient with thrombotic thrombocytopenic purpura resistant to treatment with antiplatelet drugs and plasmapheresis. Despite marked thrombocytopenia and continued treatment with aspirin, sulfinpyrazone, and dipyridamole, the urinary excretion of 2,3-dinor-thromboxane B2, a major thromboxane urinary metabolite, was within the normal range (90.3 to 368 pg/mg creatinine) at 96 pg/mg creatinine. Because of its potent antiaggregatory properties and the possibility of a defect in endogenous PGI2 production in thrombotic thrombocytopenic purpura, synthetic PGI2 (4 to 10 ng/kg-1 . min-1) was infused intravenously, first for 72 hours and then continuously for 18 days. Prostacyclin markedly reduced the excretion of 2,3-dinor-thromboxane B2, and the platelet count rose steadily to reach 100 000/mm3 by the eighth day of the second infusion. The patient remains in clinical remission, on no therapy, 7 months later. A controlled evaluation of PGI2 in thrombotic thrombocytopenic purpura is warranted. Apparent therapeutic failure in previous cases may have resulted from inadequate prolongation of PGI2 infusion.