Normal endometrium, an estrogen-responsive tissue, expresses the estrogen receptor (ER) alpha gene. Loss of ER expression, the basis for which is currently unknown, is often seen in advanced stage, poor prognosis endometrial tumors. The ER gene undergoes de novo methylation with high frequency in a wide variety of human tumors, including ER-negative breast cancers. In this study, we used several bisulfite-based detection methods to assess whether loss of ER positivity in endometrial tumors is associated with aberrant methylation of the ER gene. Although extensive methylation of a 600-bp region at the 5' end of the gene was seen in two endometrial carcinoma cell lines, none of the 55 CpGs in this region was methylated in 25 of 26 ER-deficient endometrial carcinomas.