Fifteen patients with chronic myelogenous leukemia were managed only with repeated leukapheresis for up to 26 mth. At each procedure approximately 10 l of blood were processed with a continuous flow blood cell separator over a 3 hr period. Five patients had intermittent leukapheresis (1-2 procedures/wk), and 10 of them had one or more series of intensive leukapheresis (4-5 procedures/wk). Addition of hydroxyethyl starch to the extracorporeal circuit was found to increase the removal of leukocytes fourfold. With intensive leukapheresis the leukocyte count decreased 80%, and the platelet count decreased 54% (mean values). With intermittent leukapheresis the corresponding figures were 70% and 35%. Thrombocytopenia was never clinically significant. All 15 patients experienced symptomatic improvement, and those with organomegaly had decrease in the size of the spleen and liver. Leukapheresis was not associated with morbidity, except that anemia became more severe and required periodic transfusions of packed red blood cells. Patients managed with leukapheresis did not enter bone marrow remission, and transformation of CML into blastic crisis was not prevented or delayed.