PURPOSE: We determined whether baseline urodynamic study variables predict failure after mid urethral sling surgery. MATERIALS AND METHODS: Preoperative urodynamic study variables and postoperative continence status were analyzed in women participating in a randomized trial comparing retropubic to transobturator mid urethral sling. Objective failure was defined by positive standardized stress test, 15 ml or greater on 24-hour pad test, or re-treatment for stress urinary incontinence. Subjective failure criteria were self-reported stress symptoms, leakage on 3-day diary or re-treatment for stress urinary incontinence. Logistic regression was used to assess associations between covariates and failure controlling for treatment group and clinical variables. Receiver operator curves were constructed for relationships between objective failure and measures of urethral function. RESULTS: Objective continence outcomes were available at 12 months for 565 of 597 (95%) women. Treatment failed in 260 women (245 by subjective criteria, 124 by objective criteria). No urodynamic variable was significantly associated with subjective failure on multivariate analysis. Valsalva leak point pressure, maximum urethral closure pressure and urodynamic stress incontinence were the only urodynamic variables consistently associated with objective failure on multivariate analysis. No specific cut point was determined for predicting failure for Valsalva leak point pressure or maximum urethral closure pressure by ROC. The lowest quartile (Valsalva leak point pressure less than 86 cm H2O, maximum urethral closure pressure less than 45 cm H2O) conferred an almost 2-fold increased odds of objective failure regardless of sling route (OR 2.23, 1.20-4.14 for Valsalva leak point pressure and OR 1.88, 1.04-3.41 for maximum urethral closure pressure). CONCLUSIONS: Women with a Valsalva leak point pressure or maximum urethral closure pressure in the lowest quartile are nearly 2-fold more likely to experience stress urinary incontinence 1 year after transobturator or retropubic mid urethral sling.