The use of prostacyclin infusion in thrombotic thrombocytopenic purpura is consistent with the hypothesis that patients may lack a plasma factor stimulating prostacyclin production. However, prostacyclin therapy, alone or in combination with aspirin, dipyridamole, steroid and plasmapheresis, failed in many cases. We here describe the case of a patient who responded dramatically to a combination of prostacyclin and plasma infusions, after conventional therapy had failed (plasmapheresis, fresh frozen plasma infusions). Prostacyclin was infused intravenously initially for 120 h from 4 to 9 ng kg min-1 and then continuously for 48 h at 9 ng kg min-1. Despite the scarcity of case reports in the literature, we conclude that the failure of prostacyclin in thrombotic thrombocytopenic purpura appears to be related to insufficient doses and/or duration of therapy.