Purpose: The metabolism and synthesis ofintratumoral estrogens are thought to play a very important role in the etiology and progression of endometrial carcinoma. Aromatase is a key enzyme in the conversion ofandrogens to estrogens, and aromatase localization studies have reported that aromatase immunoreactivity and mRNA were detected mainly in stromal cells. However, the effect of tumor-stromal interactions on local estrogen biosynthesis in endometrial carcinomas remains largely unknown. Experimental Design: The endometrial carcinoma cell lines (Ishikawa and RL95-2) and breast carcinoma cell line (MCF-7) were cocultured with stromal cells isolated fromendometrial carcinomas, and aromatization activity was measured using liquid chromatography-tandem mass spectrometry. We then confirmed the local biosynthesis of estrogens and tumor-stromal interactions on aromatase activity in Ishikawa and RL95-2 cells. In addition, we also examined the effects of aromatase inhibitors on cell proliferation. Results: Aromatase activity was significantly higher in cocultures with Ishikawa or RL95-2 than in each monoculture, respectively. Estrone (E1) concentrations were significantly higher than estradiol (E2) concentrations in Ishikawa and RL95-2 cells, whereas E2 was significantly higher than E 1 in MCF-7 cells. Cell proliferation was significantly inhibited in Ishikawa and RL95-2 cell cultures treated with aromatase inhibitors compared with control cultures. Conclusions: These results indicate the contribution ofnot only E2 but also E1 to cancer cell proliferation in endometrial carcinoma. Our study may provide important information on metabolism and synthesis ofintratumoral estrogens with regard to the etiology and progression ofendometrial carcinoma, thus helping to achieve improved clinical responses in patients with endometrial carcinoma, who are treated with aromatase inhibitors.