A patient with Ph1 -positive chronic myelogenous leukemia (chronic phase) had a cyclic oscillation in white blood cells, platelets and percent saturation of transferrin. The cycle comprised about 70 days. The number of circulating granulocyte-macrophage colony-forming units (CFU-GM) oscillated with the same phase, while that of bone marrow CFU-GM and erythroid colony-forming units (CFU-E) oscillated in a reverse phase. At the nadir, we observed an abnormal increase in bone marrow endogenous CFU-E (e-CFU-E). An erythropoietin (Epo) dose-response curve of CFU-E showed a high Epo-sensitivity. Anti-Epo rabbit serum did not inhibit the e-CFU-E colony formation. This indicates that Epo-independent erythropoiesis occurs periodically at the nadir. It is suggested that the interactions between the abnormal stem cell and the hematopoietic regulating system cause cyclic oscillation.