20 patients, 16 with acute or chronic leukemia and 4 with thrombocytosis,underwent 47 therapeutic cytapheresis procedures using the Fenwal CS-3000 cell separator. 16 of the 20 patients had acute clinical signs or symptoms secondary to high circulating cell counts; 14 showed symptomatic improvement following cytapheresis. An average of 2.0 whole blood volumes was processed per procedure. A mean white cell reduction of 64% and a mean platelet reduction of 53% were obtained per procedure on patients with leukemia and thrombocytosis, respectively. Hemoglobin levels decreased an average of 1.3 g/dl. Therapeutic cytapheresis procedures in which more than 1.5 blood volumes were processed did not result in significant additional cytoreduction.