ObjectiveTo evaluate the efficacy, safety, and optimum dose of once-daily oxybutynin patch for overactive bladder. MethodsA randomized double-blind trial was conducted in patients with overactive bladder symptoms for 24weeks, who received an oxybutynin patch (73.5 or 105mg) or placebo once daily for 8weeks. The primary endpoint was the change in the daily frequency of micturition from baseline to the end of study. ResultsA total of 579 patients were randomized to the placebo group (n=164), 73.5mg oxybutynin patch group (n=166), and 105mg oxybutynin patch group (n=165). The frequency of micturition (meanstandard deviation) decreased by 1.19 +/- 1.80 in the placebo group, 1.87 +/- 1.93 in the 73.5mg group, and 1.80 +/- 1.76 in the 105mg group. Compared with the placebo group, micturition decreased significantly in the 73.5mg and 105mg groups (t-test: P=0.0025 and 0.0039, respectively), while the decrease was similar in both oxybutynin groups. The oxybutynin groups showed significant improvement of urgency, urge incontinence, incontinence, nocturia, mean voided volume, and five or six domains of the King's Health Questionnaire. Dry mouth was noted in 12.1% of the 73.5mg group and 13.3% of the 105mg group. Constipation was comparable between the oxybutynin groups and the placebo group. Application site reactions were less frequent in the 73.5mg group than the 105mg group. Conclusion: The efficacy of oxybutynin patch was confirmed, and the optimum dose for Japanese patients with overactive bladder was 73.5mg.