Studies were done in order to assess the effectiveness of repeated leukapheresis therapy for patients with chronic myelogenous leukemia (CML), since chemotherapy may be unnecessary and even harmful in a limited number of CML patients. For this purpose, 10 patients with CML were treated with leukapheresis for periods ranging from 1 to 24 mth. For each procedure, an average of 1.1 x 1011 (110 gm) leukocytes were removed from 10 liters of blood processed on a continuous flow basis with the blood cell separator over an average of 3 hr; good long term control was achieved in 7 patients.