OBJECTIVES: This study identifies and corrects the bias in previous reports comparing tolterodine and oxybutinin XL. By adjusting for different patient populations used, the current study provides updated results. METHODS: Using recent clinical trials, correction factors were calculated to remove bias from previous efficacy estimates. That bias resulted from failure to reconcile differences in the separate studies for the two drugs. Oxybutinin XL trials included known responders, all with urinary incontinence (UI), whereas tolterodine trials examined both na ve and previously treated patients with overactive bladder (OAB), some without UI. Therefore, measures of incontinence from the oxybutinin XL trials were adjusted to levels appropriate for a na ve and previously treated OAB population. To assess and correct the effect of the bias, cost-effectiveness results were recalculated based on corrected efficacy. The pharmacoeconomic model from the previous analysis was not described in sufficient detail to be used. Instead, it was simplified to generate revised cost-effectiveness measures using resource and cost data from the previous analysis. RESULTS: When adjusted, complete continence was 22.9% for oxybutinin XL and 24.6% for tolterodine. The simplified model then estimated 6-month treatment costs to be $1,684 for oxybutinin XL and $1,661 for tolterodine. Estimated costs per continent day were $45 and $42, respectively. CONCLUSIONS: Cost-effectiveness analysis can be misleading unless biases are explicitly identified and corrected. After adjusting for limitations in the methodology previously used, no significant differences in cost effectiveness remain between oxybutinin XL and tolterodine. The corrected results are consistent with findings from trials directly comparing oxybutinin XL and tolterodine, which also found no efficacy differences.