To evaluate the accuracy of Transrectal Sonography (TRS) and a new technique, Transvaginal Sonography with Water-Contrast in the Rectum (RWC-TVS), in the diagnosis of rectosigmoid endometriosis, and the accuracy of Barium Enema (BE) and RWC-TVS in the detection of intestinal stenosis due to endometriosis. In a prospective study, we compared the findings of TRS and RWC-TVS performed before surgery with the operative and pathologic findings in 61 consecutive patients who underwent laparoscopy or laparotomy for suspected rectosigmoid endometriosis. The accuracy of BE and RWC-TVS in the detection of intestinal stenosis was evaluated comparing the radiologic and ultrasonographic results with the macroscopic findings at surgery and pathology. RWC-TVS diagnosed rectosigmoid endometriosis with the same accuracy of TRS and was equally efficient as BE in the detection of a significant intestinal lumen stenosis. For the diagnosis of rectosigmoid endometriosis the sensitivity, specificity, positive and negative predictive values of TRS and RWC-TVS were 88.2% and 96%, 80%, and 90%, 95.7%, and 98%, and 57.1% and 81.8%, respectively. For the detection of intestinal stenosis the sensitivity, specificity, positive and negative predictive values of BE and RWC-TVS were 93.7% and 87.5%, 94.2% and 91.4%, 88.2% and 82.3%, and 97% and 94.1%, respectively. RWC-TVS is a new, simple technique for a single-step and accurate preoperative assessment of rectosigmoid endometriosis.