Purpose : Long-term persistence with pharmacotherapy for overactive bladder (OAB) requires a drug with an early onset of action and good efficacy and tolerability profile. Although antimuscarinics improve OAB symptoms within 1–2 weeks of initiating treatment, adherence after 3 months is relatively poor due to bothersome side effects (e.g., dry mouth and constipation). Mirabegron, a β3-adrenoceptor agonist, has demonstrated significant improvements in key symptoms of OAB and good tolerability after 12 weeks in Phase III studies. ; Methods : This was a prespecified pooled analysis of three randomized, double-blind, placebo-controlled, 12-week studies, and a Phase II study, to evaluate efficacy and tolerability of mirabegron 25 and 50 mg versus placebo. The main efficacy endpoints were change from baseline to week 1 (Phase II only), week 4, and final visit in mean number of incontinence episodes/24 h, micturitions/24 h, and mean volume voided/micturition (MVV). ; Results : A significant benefit for mirabegron 25 and 50 mg versus placebo was evident at the first assessment point, 4 weeks after initiation of therapy, in Phase III studies for incontinence, micturitions, and MVV. The earliest measured benefit was after 1 week, in the Phase II study. Quality-of-life parameters also significantly improved with mirabegron 25 and 50 mg as early as week 4. Significant benefits continued throughout the studies. Mirabegron was well tolerated. ; Conclusions : The early onset of action and good overall efficacy and tolerability balance that mirabegron offers may lead to high rates of persistence with mirabegron in the long-term treatment of OAB.