Cold-inducible RNA-binding protein (CIRP), an 18-kD protein in the mouse and human, is induced by lowering the temperature of cultured cells. CIRP is possibly a cell cycle regulator because its overexpression results in prolongation of G1 phase in vitro. We investigated the immunohistochemical expression of CIRP in 39 endometrial carcinomas, 12 endometrial hyperplasias, and 27 normal endometria using polyclonal antibody against CIRP and confirmed by Western blot analysis. CIRP was localized in the nuclei of glandular, stromal, and endothelial cells. The intensity of CIRP expression in glandular cells during the menstrual cycle was inversely proportional to its proliferative (Ki-67) activity, whereas it remained unchanged in stromal and vascular endothelial cells. The intensity of CIRP expression in hyperplastic glands was variable, whereas CIRP expression was absent or markedly reduced in most of the endometrial carcinomas. These results suggest that CIRP may participate in the cell cycle regulation of normal endometrium and the loss of its expression may be involved in endometrial carcinogenesis.