OBJECTIVE Endoscopic ultrasound provides accurate information about the anatomy of the anal sphincter. The purposes of this study were to evaluate the use of flexible echo endoscopes to examine the anal sphincters, to validate the diagnosis of internal and external sphincter defects obtained using echo endoscopes by comparison with surgical findings, and to assess the outcome after surgical sphincter repair. METHODS Twenty-eight patients (13 women, 15 men, median age 50 years, range 30-83) with fecal incontinence--which was of traumatic origin in all but one (childbirth: n = 8; anorectal surgery: n = 17; biopsy of the prostate: n = 2; no trauma: n =1)--were prospectively investigated by endosonography using an echo colonoscope (n = 14) or an echo gastroscope (n = 14) (CF-UM20, GF-UM20, Olympus Optical). The location and extent of the defects of the internal or external sphincters, or both, were compared with the surgical findings in all patients. The surgical outcome was defined as excellent, improved, or unchanged. RESULTS At surgery, 25 of the 28 patients had an isolated internal sphincter defect (n = 15) or combined sphincter defect (n = 10). Endoscopic ultrasound identified all of the external anal sphincter defects (n = 10), and correctly excluded a defect in 15 of 18 patients (sensitivity, specificity, and accuracy 100%, 83% and 89%, respectively). All of the internal sphincter defects (n = 25) were detected by endosonography. In three patients, a postulated intact internal sphincter was confirmed by surgery (accuracy 100%). In two patients, the extent of the sphincter defect was underestimated. Despite good visualization of the internal and external anal sphincters, as well as of the puborectal muscle in all patients, the shape, diameter, and full radial image sector (360 degrees) of the echo gastroscope made this instrument more practicable than the echo colonoscope. Nineteen of 25 patients who underwent surgery (76%) with proved sphincter defects experienced improvement, the figure reaching 87% (13 of 15) in patients who received isolated internal sphincter defect repair. CONCLUSIONS Anal endosonography, even using flexible echo endoscopes, is an accurate method for identifying anal sphincter defects, and is the method of choice for preoperative sphincter mapping with special regard to internal sphincter repair, which can be carried out with excellent results.