We report on the outcome of six consecutive adult patients who presented with microangiopathic anemia and thrombocytopenia. Clinical parameters on admission included platelet counts less than 45,000/mm3, microangiopathic red blood cell morphology, mental status abnormalities, and in three, rapidly progressive azotemia requiring dialysis. All patients underwent plasma exchange therapy as part of their treatments. Patients with renal failure underwent plasma exchange with a hollow fiber plasma separator, while those without renal failure were treated with a cytocentrifuge. All received fresh frozen plasma as replacement solution and were treated with glucocorticoids as well. For all six patients, plasmapheresis and conventional drug therapy resulted in remission that has lasted for 16 +/- 5 months (range 8 to 24 months). Early cessation of plasmapheresis in two patients resulted in rapid relapse. Patients who required dialysis now have a mean creatinine of 2.0 +/- 0.9 mg/dL (range 1.2 to 3.5). With similar volumes of exchange, and the same number of treatments, less fresh frozen plasma was used in the three patients treated with the hollow fiber separator than in patients treated with the cytocentrifuge (6.3 +/- 3.7 v 14.8 +/- 4.3 U/exchange, P less than 0.05). We conclude that plasmapheresis is a useful therapeutic modality for the treatment of thrombocytopenia and microangiopathic hemolytic anemia. In addition the use of a hollow fiber plasma separator for plasmapheresis is safe and efficient, particularly when concurrent dialysis is required.