Given that prolonged exposure to unopposed estrogen has been implicated in endometrial carcinogenesis, our goal was to evaluate the effect of As(2)O(3) on regulation of estrogen receptor-alpha (ERa) expression in endometrial cancer cells. As(2)O(3) inhibited ER- mRNA and protein expression in a dose-dependent manner in both the Ishikawa and ECC-1 endometrial cancer cell lines. Treatment with As(2)O(3) resulted in rapid phosphorylation of the p42/p44 MAPK which could be abolished by addition of the MAPK inhibitor, U0126. Although treatment with U0126 alone resulted in up-regulation of ER- mRNA and protein, exposure to U0126 in combination with As(2)O(3) counteracted As(2)O(3)'s inhibitory effect on ER- expression. We provide evidence that As(2)O(3) inhibits ER- mRNA and protein expression in endometrial cancer cells, potentially through interaction with the MAPK pathway. Thus, As(2)O(3) may be valuable for its anti-estrogenic activity in combination with its anti-tumorigenic effects and be a novel therapeutic agent for endometrial cancer.