Objective: To investigate mRNA expression of aromatase and 17β - hydroxysteroid dehydrogenase type 2 (17β HSD2) in epithelial and stromal cells from eutopic and ectopic endometrium of patients with deep endometriosis. Design: Prospective study. Setting: University hospital. Patient(s): Patients with deep endometriosis and fertile women with macroscopically normal pelvic cavities. Intervention(s): During surgery,30 endometrial and 16 endometriotic samples were obtained from 30 patients with deep endometriosis. Control endometrial samples were obtained from 24 fertile women with macroscopically normal pelvic cavities who underwent laparoscopic tubal ligation or reversal of tubal sterilization. Epithelial cells and stromal cells from endometrial or endometriotic tissues were microdissected using laser capture microdissection. Main Outcome Measure(s): Expression levels of aromatase and 17β HSD2 mRNA in microdissected epithelial and stromal cells were determined using quantitative real- time reverse transcriptase polymerase chain reaction. Result(s): Aromatase mRNA expression was significantly higher in epithelial cells than in stromal cells in both eutopic and ectopic endometrium obtained from endometriosis patients. In the ectopic endometrium of 8 patients (8/16, 50% ), 17β HSD2 expression was not detected in either epithelial or stromal cells. In eutopic endometrium from endometriosis patients, 17β HSD2 expression in epithelial cells was significantly increased during the early, middle, and late secretory phases compared with the late proliferative phase, whereas no significant cyclical difference was detected in control endometrium. Conclusion(s): Local estrogen concentration may be much higher in epithelial cells than in stromal cells in deep endometriotic tissue. Objective: To investigate mRNA expression of aromatase and 17β-hydroxysteroid dehydrogenase type 2 (17β HSD2) in epithelial and stromal cells from eutopic and ectopic endometrium of patients with deep endometriosis. Design: Prospective study. Setting: University hospital. Patient (s): Patients with deep endometriosis and fertile women with macroscopically normal pelvic cavities. Intervention (s): During surgery, 30 endometrial and 16 endometriotic samples were obtained from 30 patients with deep endometriosis. Control endometrial samples were obtained from 24 fertile women with macroscopically normal pelvic cavities who underwent laparoscopic tubal ligation or reversal of tubal sterilization. Epithelial cells and stromal cells from endometrial or endometriotic tissues were microdissected using laser capture microdissection. Main Outcome Measure (s): Expression levels of aromatase and 17β HSD2 mRNA in microdissected epithelial and stromal cells were determined using quantitative real- time reverse transcriptase polymerase chain reaction. Results: Aromatase mRNA expression was significantly higher in epithelial cells than in stromal cells in both eutopic and ectopic endometrium obtained from endometriosis patients. In the ectopic endometrium of 8 patients (8/16, 50% ), 17β HSD2 expression was not detected in either epithelial or stromal cells. In eutopic endometrium from endometriosis patients, 17β HSD2 expression in epithelial cells was significantly increased during the early, middle, and late secretory phases compared with the late proliferative phase, significant cyclical difference was detected in control endometrium. Conclusion (s): Local estrogen concentration may be much higher in epithelial cells than in stromal cells in deep endometriotic tissue.