To clarify the roles of estrogen receptors (ERs) in gastric cancers, we evaluated expression of ER-alpha and ER-beta mRNAs in 41 pairs of tumorous and non-tumorous tissues of gastric cancer patients and in six gastric cancer cell lines by reverse transcription-polymerase chain reaction method. ER-alpha and ER-beta mRNAs were detected in 21 (51%) and 30 (73%) of 41 tumors and in 15 (37%) and 36 (88%) of 41 corresponding normal tissues, respectively. There were no statistically significant associations between expression of ER-alpha and/or ER-beta mRNAs in tumors and clinicopathologic factors. Between the tumorous and normal tissues, expression of ER-alpha and ER-beta mRNAs were changed in 20 (49%) and unchanged in 21 (51%) of the 41 cases. The incidences of lymph node metastasis and liver metastasis were significantly higher in changed cases than in unchanged cases (P=0.031 and P=0.021, respectively). We confirmed that ER-alpha and ER-beta mRNA were expressed in 2 and 6 of the six gastric cancer cell lines, respectively. Together with this finding, our results indicate that ER-beta mRNAs are preferentially expressed in gastric cancers. Our data also suggest that altered expression of ER-alpha and ER-beta mRNAs in tumors compared with corresponding normal gastric tissues is related to increased metastatic potential in gastric cancers. Further studies are needed to clarify the role of ERs in gastric cancers.