Chronic eosinophilic leukemia (CEL) is a rare myeloproliferative disease in which autonomous, clonal proliferation of eosinophilic precursors results in persistent increase of eosinophils in the blood and bone marrow. A case of CEL spontaneous oscillation of white blood cell (WBC) count is presented. The cycle of WBC variation comprised about 60 days. Similar cyclic variations were noted in his platelet count, hemoglobin level and bone marrow cellularity, as well as in the spleen size, which was directly correlated with the WBC count. The numbers of bone marrow erythroid colony-forming units (CFU-E), granulocyte-macrophage colony-forming units (CFU-GM) and the serum level of colony-stimulating factors (CSFs) were also regularly changed during the oscillation of WBC. Bone marrow hyperplasia was accompanied with the increase in peripheral WBC count, suggesting that the variation of cell production caused the cyclic oscillation.