Described in this study are 38 patients who received treatment for thrombotic thrombocytopenic purpura in 15 hospitals in Israel and the New York City area since 1977, when plasma therapy was introduced. Thirty-seven patients received plasma therapy and 30 survived. In 12 patients (37 percent of survivors), relapsing thrombotic thrombocytopenic purpura developed, manifested by thrombocytopenia and microangiopathic hemolytic anemia, and less frequently by neurological or renal abnormalities. Six patients had a second relapse, and two had five relapses. To assess the severity of the disease, a scoring system was designed based on the four major manifestations of thrombotic thrombocytopenic purpura listed above. The patients who died had a significantly higher score than those who survived. The initial episodes and the relapses of patients with relapsing thrombotic thrombocytopenic purpura were milder than those in patients who only had a single episode and survived. Two relapses, however, were fatal, demonstrating that relapsing thrombotic thrombocytopenic purpura is not a benign disorder. Infections, pregnancy and surgery were frequently associated with the initial episodes and the relapses. Hence, patients who recover from thrombotic thrombocytopenic purpura should be alerted to the possibility of relapse in association with these conditions.