A computerized morphometrical investigation was performed on endometriotic tissue from the peritoneum (n = 225) and rectovaginal nodules (n = 65) to compare histologically and stereologically the rectovaginal septum endometriotic nodule to peritoneal endometriosis. Mitotic activity, stromal vascularization and the epithelium/stroma ratio were found to be significantly different in peritoneal and rectovaginal endometriosis. The evaluation revealed a major role of glandular epithelium in rectovaginal nodules where the stroma sometimes appeared absent around glandular epithelium. The study demonstrated opposite effects of gonadotrophin-releasing hormone agonists (GnRHa) and lynestrenol on the two lesions. Indeed, in peritoneal endometriosis, after GnRHa therapy, our study demonstrated a lower rate of mitosis and poor stromal vascularization. The same drug was unable to induce the same effects in the nodule although, in contrast, lynestrenol has a strong effect on nodule vascularization. In conclusion, it is suggested that the rectovaginal adenomyotic nodule is a specific disease, different from peritoneal endometriosis. It is not the consequence of 'deep infiltrating' endometriosis but can probably develop from Mullerian rests present in the rectovaginal septum.