Androgens mediate their effects through the androgen receptor (AR) and have antiproliferative effects on uterine endometrial cells. In this report, we investigated methylation status and the expression of the AR gene in normal endometrium and uterine endometrial cancer (UEC) tissues using methylation-specific polymerase chain reaction (MSP) and immunohistochemical staining. Seventy of 89 cancer samples were AR negative, although 39 of 46 normal samples were AR positive by immunohistochemistry. By MSP, 64 of 89 cancer samples showed only methylated AR alleles, although all normal tissues showed both unmethylated and methylated AR alleles. To determine whether similar changes occurred in methylation status in the UEC carcinogenesis, we studied AR methylation using pairs of cancerous and normal samples from 28 patients. Twenty-three of 28 cancer samples showed only methylated AR alleles, although all normal samples showed both unmethylated and methylated alleles. All of the 23 cancer samples that lost unmethylated alleles were negative for AR by immunohistochemical analysis. Reverse transcription-polymerase chain reaction was performed by using UEC cell lines with and without treatment by the demethylating reagent 5-aza-2'-deoxycytidine. No AR expression was found in any of the UEC cell lines, except for MFE-296 without 5-aza-2'-deoxycytidine. Treatment with 5-aza-2'-deoxycytidine restored AR expression in all of the UEC cell lines that showed no AR expression before treatment. This study is the first to report that the possible mechanism of AR inactivation in endometrial cancer is through hypermethylation of the AR gene CpG islands.