Phenytoin removal by plasmapheresis was evaluated in a 17-year-old girl with thrombotic thrombocytopenia purpura. Free and total phenytoin concentrations were measured in the patient's serum and in the plasma removed by plasmapheresis. Plasmapheresis was performed on three separate days with the removal of 4.7%, 3.3%, and 2.7% of total body stores. Free phenytoin concentration was similar in both the plasma removed by plasmapheresis and the patient's serum. Plasmapheresis did not significantly alter the serum concentration of phenytoin; dosage adjustments of phenytoin are therefore unnecessary.