We report a case of chronic myelocytic leukemia (CML) with cyclic oscillation of platelet and leukocyte counts and attempt to elucidate the oscillatory mechanism from the standpoint of cytokine regulation of hematopoiesis. A 57-year-old woman with a diagnosis of CML exhibited platelet and white blood cell (WBC) count fluctuations of a cyclic nature. The average duration of the cycles was about 8 weeks. The patient suffered from headache, fatigue, and malaise at the peak of the cycle. The peak thrombopoietin concentration in peripheral blood coincided with a period of decrease in platelet numbers. The change in transforming growth factor beta (TGF-beta) level paralleled that of the platelet numbers. A progenitor cell assay revealed the suppression of trilineage colony formation in the presence of plasma from the blood cell peak point, and this suppression was completely blocked when the plasma was incubated with an anti-TGF-beta antibody. From these findings, we concluded that the cyclic oscillation of the platelet, WBC, and reticulocyte counts had been induced by excess negative feedback to megakaryopoiesis by TGF-beta.