BACKGROUND: Midurethral slings are increasingly used for the treatment of stress incontinence, but there are limited data comparing types of slings and associated complications. METHODS: We performed a multicenter, randomized equivalence trial comparing outcomes with retropubic and transobturator midurethral slings in women with stress incontinence. The primary outcome was treatment success at 12 months according to both objective criteria (a negative stress test, a negative pad test, and no retreatment) and subjective criteria (self-reported absence of symptoms, no leakage episodes recorded, and no retreatment). The predetermined equivalence margin was ±12 percentage points. RESULTS: A total of 597 women were randomly assigned to a study group; 565 (94.6%) completed the 12-month assessment. The rates of objectively assessed treatment success were 80.8% in the retropubic-sling group and 77.7% in the transobturatorsling group (3.0 percentage-point difference; 95% confidence interval [CI], -3.6 to 9.6). The rates of subjectively assessed success were 62.2% and 55.8%, respectively (6.4 percentage-point difference; 95% CI, -1.6 to 14.3). The rates of voiding dysfunction requiring surgery were 2.7% in those who received retropubic slings and 0% in those who received transobturator slings (P = 0.004), and the respective rates of neurologic symptoms were 4.0% and 9.4% (P = 0.01). There were no significant differences between groups in postoperative urge incontinence, satisfaction with the results of the procedure, or quality of life. CONCLUSIONS: The 12-month rates of objectively assessed success of treatment for stress incontinence with the retropubic and transobturator approaches met the prespecified criteria for equivalence; the rates of subjectively assessed success were similar between groups but did not meet the criteria for equivalence. Differences in the complications associated with the two procedures should be discussed with patients who are considering surgical treatment for incontinence. (ClinicalTrials.gov number, NCT00325039.). Copyright © 2010 Massachusetts Medical Society.