BACKGROUND: Women without stress urinary incontinence undergoing vaginal surgery for pelvic-organ prolapse are at risk for postoperative urinary incontinence. A midurethral sling may be placed at the time of prolapse repair to reduce this risk. METHODS: We performed a multicenter trial involving women without symptoms of stress incontinence and with anterior prolapse (of stage 2 or higher on a Pelvic Organ Prolapse Quantification system examination) who were planning to undergo vaginal prolapse surgery. Women were randomly assigned to receive either a midurethral sling or sham incisions during surgery. One primary end point was urinary incontinence or treatment for this condition at 3 months. The second primary end point was the presence of incontinence at 12 months, allowing for subsequent treatment for incontinence. RESULTS: Of the 337 women who underwent randomization, 327 (97%) completed follow-up at 1 year. At 3 months, the rate of urinary incontinence (or treatment) was 23.6% in the sling group and 49.4% in the sham group (P<0.001). At 12 months, urinary incontinence (allowing for subsequent treatment of incontinence) was present in 27.3% and 43.0% of patients in the sling and sham groups, respectively (P=0.002). The number needed to treat with a sling to prevent one case of urinary incontinence at 12 months was 6.3. The rate of bladder perforation was higher in the sling group than in the sham group (6.7% vs. 0%), as were rates of urinary tract infection (31.0% vs. 18.3%), major bleeding complications (3.1% vs. 0%), and incomplete bladder emptying 6 weeks after surgery (3.7% vs. 0%) (P≤0.05 for all comparisons). CONCLUSIONS: A prophylactic midurethral sling inserted during vaginal prolapse surgery resulted in a lower rate of urinary incontinence at 3 and 12 months but higher rates of adverse events. (Funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Institutes of Health Office of Research on Women's Health; OPUS ClinicalTrials.gov number, NCT00460434.).