A solid-phase immunoradiometric assay for serum ferritin has been developed and applied to 30 patients with chronic myeloid leukemia (CML), including 13 patients with a typical chronic phase, 12 with bone marrow fibrosis without evidence of an acute phase and 5 with an acute phase. Patients were divided in to three subgroups according to the erythrokinetic data as studied by the 59iron kinetic test. Serum ferritin was clearly correlated with the quality of erythropoiesis but not with blast cell mass nor white cell count. Given the prognostic value of dyserythropoiesis in the course of CML, serum ferritin determination may be a helpful and simple parameter in the follow-up of CML patients.