Objective This planned secondary analysis of the Outcomes Following Vaginal Prolapse Repairs and Midurethral Sling trial assessed whether treatment knowledge differed between randomized groups at 12 months and whether treatment success was affected by treatment perception. Study Design Sham suprapubic tension-free vaginal tape (TVT) incisions were made in the Outcomes Following Vaginal Prolapse Repairs and Midurethral Sling trial participants randomized to no-TVT. Primary surgical outcomes and maintenance of blinding was assessed at 12 months. Knowledge of treatment assignment was compared between groups, and the relationship with treatment success rates was assessed. Results Prior to the 12 month postoperative visit, only 4% of treated participants (13 of 336) formally reported unmasking. At 12 months, 94% of the randomized participants (315 of 336) provided treatment knowledge data. Sixteen TVT participants (10%) reported treatment knowledge; most (n = 15, 94%) were correct; 17 of the sham participants (11%) reported treatment knowledge; half (n = 8, 47%) were correct. Similar proportions of unmasked participants who reported no treatment knowledge correctly guessed/perceived treatment assignment (sham, 46 [33%] vs TVT, 44 [33%]). We did not detect significant differences in treatment success rates based on perception within and across received treatment groups (perceived sham vs TVT overall [P =.76]). Of those receiving TVT, more participants perceiving TVT had treatment success compared with those who perceived sham (84% vs 74%; P =.29). Among sham participants, more participants perceiving sham had success compared with those who perceived receiving TVT (65% vs 56%; P =.42). Conclusion Sham surgical incisions effectively mask TVT randomization. These findings may help to inform future surgical trial designs.