Objective To assess the accuracy of transvaginal sonography (TVS) in defining size and location of deep infiltrating endometriosis (DIE) with laparoscopic/histologic confirmation. Design Prospective observational study. Setting University hospital. Patient(s) One hundred four women with suspected DIE on the basis of TVS. Intervention(s) Patients with DIE underwent TVS evaluation before laparoscopic surgery. An accurate mapping of the extent of the disease was recorded during TVS and at laparoscopy. This new mapping system was developed to assess the extent of endometriosis by measuring the size and depth of the lesions at the various pelvic locations. Main Outcome Measure(s) Surgical and histologic confirmation of the ultrasonographic data to evaluate the presence and location of DIE and creation of a new mapping methodology for detecting DIE by TVS. Result(s) Depending on the different location of the lesions, the accuracy of TVS ranged from 76%–97%. The lowest sensitivity (59%) and accuracy (76%) were obtained for TVS in the diagnosis of vaginal endometriosis, whereas the greatest accuracy (97%) was shown in detecting bladder lesions and Douglas obliteration. Conclusion(s) This new ultrasound mapping system is accurate for detecting the extent of DIE and may be useful for preoperative planning and intraoperative management of symptomatic patients with DIE.