The spontaneous incidence of myeloid leukemia in female mice was slightly higher than in male mice, whereas the radiation-induced incidence was significantly lower than in male mice. We also examined whether the incidence of myeloid leukemia was related to inflammatory response. Mice had a piece of cellulose acetate membrane inserted into the peritoneal cavity to cause inflammation. This did not affect the incidence of myeloid leukemia in unirradiated mice at all, but in 2.84 Gy irradiated mice the incidence (35.9% in male, 26.0% in female mice) increased significantly compared with irradiated-only mice (23.9% and 12.0%, respectively). From these results, the physiological fluctuation of humoral factors by means of inflammatory response is considered to increase the development of radiation-induced myeloid leukemia.