Endometriosis is an estrogen-dependent gynecological disease causing pelvic pain and infertility. Impaired estrogen metabolism is thought to play a pivotal role in the pathogenesis of the disease. While there is some information on factors involved in the synthesis of E2, information on E2-deactivating enzymes is still very limited. To elucidate the intracrinology of endometriotic tissues, the authors analyze the expression of aromatase and E2-inactivating estrogen sulfotransferase (EST) in paired biopsies obtained simultaneously from the endometrium and from endometrial lesions of each of 35 patients with peritoneal or ovarian endometriosis and in cycling endometria from 33 women without endometriosis. Protein localization was demonstrated by immunohistochemistry. Aromatase expression was found in endometriotic glands in 32 of 35 cases and was elevated in comparison to corresponding uterine endometria (25 of 35 cases, P = .021, chi(2) test). The difference was even more pronounced when uterine endometria from endometriosis patients were compared with that of healthy controls (8 of 33 cases, P < .001, chi(2) test). The EST levels were essentially unchanged. The elevated expression of aromatase in eutopic and ectopic endometrium from patients with endometriosis in the presence of comparable EST provides further evidence for unopposed local biosynthesis of estrogens in endometriosis.