PURPOSE: Overlapping sphincteroplasty is the surgery of choice for incontinent patients with an anterior defect after childbirth. Numerous predictive factors have been proposed, but no preoperative variables have been successfully shown to be reproducible. The purpose of this study was to assess the prognostic value of voluntary contraction of the puborectal sling before sphincter repair for anal incontinence. METHODS: This prospective study evaluated 109 consecutive women who underwent surgery for postobstetric anal incontinence. Voluntary contraction of the puborectal sling was measured by perineal ultrasound before the surgery. Severity of anal incontinence was evaluated preoperatively and postoperatively with the Miller Incontinence Score (total incontinence = 18, complete continence = 0). RESULTS: The proportion of patients with scores ≤ 3 was 16.7 percent when the preoperative voluntary contraction of the puborectal sling was ≤ 4 mm, 48.1 percent when it was 4.1 to 8 mm, and 98.7 percent when it was >8 mm (P < 0.001). Using ≤ 8 mm to define abnormal shortening, the sensitivity of the test was 0.95 (95 percent confidence interval, 0.75-1.00) and specificity was 0.84 (95 percent confidence interval, 0.75-0.91). CONCLUSION: A preoperative voluntary contraction of the puborectal sling >8 mm convincingly discriminates between patients with a good functional outcome and those with an unsatisfactory outcome after sphincter repair for postobstetric anal incontinence.