OBJECTIVE: To characterize the urodynamic (UDS) changes in subjects 24 months after Burch urethropexy and autologous fascial sling surgery for stress urinary incontinence. METHODS: In the Stress Incontinence Surgical Treatment Efficacy Trial (SISTEr), 655 women underwent standardized UDSs before and 2 years after Burch or sling surgery. Paired t tests were used to compare the pre- and postoperative UDS measures by treatment group. Analysis of variance models were fit predicting the change in UDS measures, controlling for the treatment group. RESULTS: The noninstrumented maximal flow rate decreased 3.6 mL/s in the Burch group and 4.7 mL/s in the sling group (P = .42). The average flow rates also decreased (2.4 mL/s in the Burch group and 3.8 mL/s in the sling group, P = .039). No difference was found in the increases in first sensation between the Burch and sling groups (23.3 and 29.3 mL, respectively, P = .61). Also, no differences were found in the reduction in the pressure flow study maximal flow rates (2.3 mL/s in the Burch group and 4.4 mL/s in the sling group, P = .11). An increased detrusor pressure at maximal flow rate (11.4 cm H(2)O, P < .001) was seen only after the sling procedure. Increases in the bladder outlet obstruction index occurred after both procedures, with greater increases seen after sling surgery (change, Burch +6.27 vs sling +20.12, P = .001). CONCLUSION: The Burch colposuspension and autologous fascial sling procedures were associated with similar decreases in noninstrumented flow rates, and the sling was associated with greater increases in the detrusor pressure at maximal flow rate and bladder outlet obstruction index. These changes suggest that both procedures are effective, in part, because of increased outlet resistance. However, the sling procedure might be more obstructive.