The response to intensive plasmapheresis was evaluated in 13 patients (7 males, 6 females; median age 28 years) with microangiopathic thrombotic syndromes (MTS) who were treated at our center during the last 10 years. The most common initial clinical features were anemia, hemorrhage and neurologic abnormalities. Twelve patients had severe thrombocytopenia and 8 had renal failure. Fourteen plasmapheresis courses were carried out to treat 13 initial episodes and one relapse. The mean volumes of removed plasma and fresh frozen plasma infused at each exchange were 57 ml/kg and 41.7 ml/kg, respectively. Eight patients received antiplatelet drugs and/or corticosteroids. After a mean number of 10.6 exchanges per patient and course, complete remission (CR) was achieved in 7 cases (6 initial episodes and one relapse) and a partial response in 3. Four patients did not respond to therapy and died from progression of the disease. The early institution of plasmapheresis and the rapidity to respond were the only factors significantly associated with the achievement of CR.