BACKGROUND Transvaginal ultrasonography (TVUS) has important advantages compared with transrectal ultrasonography (TRUS): it is less invasive, is cost-effective, is a familiar and well-accepted approach, and anesthesia is not required. We compared the accuracy of TVUS and TRUS for diagnosing rectal wall involvement in patients presenting with histologically proved deeply infiltrating endometriosis (DIE). METHODS Prospective study of 134 patients with histologically proved DIE underwent preoperative investigations using both TVUS and TRUS. The radiologist (TVUS) and sonographer (TRUS) were unaware of the clinical findings but knew that DIE was suspected. RESULTS DIE was confirmed histologically for all the patients. A rectal wall involvement was histologically proved for 75 patients (56%). For the diagnosis of infiltration of the intestinal wall, TVUS and TRUS, respectively, had a sensitivity of 90.7% and 96.0%, a specificity of 96.5% and 100.0%, a positive predictive value of 97.1% and 100.0% and a negative predictive value of 88.9% and 95.2%. CONCLUSIONS TVUS and TRUS have similar degrees of accuracy for predicting intestinal involvement. TVUS must be the first-line imaging process to perform for patients presenting with clinically suspected DIE. The question for the coming years is to define if it is necessary for TRUS to be carried out systematically in cases of clinically suspected DIE.