In view of the potential of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] as a cell-differentiation-inducing agent in endometrial cancer, the localization of the vitamin D receptor (VDR) was examined immunohistochemically in 21 endometrial adenocarcinoma specimens, and the effect of 1,25(OH)2D3 on cell growth, as well as the phenotypic changes for cell maturation after treatment with 1,25(OH)2D3, was investigated in 2 endometrial carcinoma cell lines (AMEC-1, RL95-2). The VDR was detected in 14 of the 21 endometrial carcinoma specimens. The growth of RL95-2 cells expressing VDR was inhibited to 44% when cultured with 50 nM 1,25(OH)2D3 for 6 days. In contrast, the growth of AMEC-1 cells not expressing VDR was completely uninhibited even when cultured with 100 nM 1,25(OH)2D3 for 6 days. The RL95-2 cells exposed to 50 nM 1,25(OH)2D3 for 6 days had an increasing expression for 52.5 kD or 45 kD cytokeratin polypeptide, and they became columnar with pronounced polarity and formed gland-like structures when cultured in collagen gel. These results suggest that endometrial adenocarcinoma is a target for 1,25(OH)2D3, which appears to function as a cell-differentiation-inducing agent for the treatment of endometrial cancer.