['To compare histologically and stereologically the endometriotic nodule of the rectovaginal septum to peritoneal endometriosis.', 'Morphometric investigation, cytokeratin and vimentin content, and steroid receptor evaluation were performed on endometriotic tissue from the peritoneum (n = 52) and rectovaginal nodules (n = 68).', 'An academic teaching hospital.', 'Biopsies were taken from 120 patients undergoing a laparoscopy for infertility and/ or pelvic pain (52 from typical black peritoneal endometriotic implants and 68 from endometriotic nodule of the rectovaginal septum). None of the patients were treated.', 'Mitotic activity was found to be significantly different in peritoneal and rectovaginal endometriosis. The evaluation suggested that the stroma is not mandatory for the invasion of glandular epithelium in the rectovaginal nodule, which is, like a adenomyoma, a circumscribed nodular aggregate of smooth muscle and glandular elements. Cytokeratin and vimentin content as well as the estrogen receptor (ER) and P receptor (PR) content were significantly lower in both types of lesion when compared with eutopic endometrium. But vimentin immunoreactivity in epithelium, as well as the ER and PR content, were significantly lower in nodules when compared with black peritoneal lesions.'] Are peritoneal endometriosis and `` endometriotic '' nodules of the rectovaginal septum two different entities?