To evaluate the efficacy, safety/tolerability, and dose-response relationship of imidafenacin in Japanese patients with overactive bladder. Men and women who had overactive bladder symptoms were randomized equally to double-blind treatment with 0.05, 0.1, or 0.25 mg of imidafenacin twice daily or a placebo for 12 weeks, and assessed for efficacy and safety. Overall, 401 patients were enrolled and randomized for treatment with 0.1 mg of imidafenacin/day (99 patients), 0.2 mg of imidafenacin/day (100), 0.5 mg of imidafenacin/day (101), or a placebo (101). After 12 weeks of treatment, the number of incontinence episodes was reduced in a dose-dependent manner, and a significant difference between the imidafenacin treatment and the placebo was observed (P < 0.0001). Compared with the placebo, imidafenacin caused significant reductions in urgency incontinence, voiding frequency, and urinary urgency, and a significant increase in the urine volume voided per micturition. Imidafenacin was also well tolerated. The incidence of dry mouth in the imidafenacin groups increased dose-dependently. Even though the percentage of patients receiving 0.5 mg/day who discontinued treatment due to dry mouth was high (8.9%), the percentages in the 0.1 mg/day and 0.2 mg/day groups (1.0% and 0.0%, respectively) were comparable with that in the placebo group (0.0%). Considering the balance between efficacy and safety, 0.1 mg of imidafenacin twice daily appeared to be a clinically appropriate dose for treating overactive bladder. This dose was therefore selected for further evaluation in large-scale phase III studies.