Objectives: To investigate safety, tolerability and efficacy of long-term (52 weeks) open-label treatment with mirabegron 50 mg, with an optional dose increase to 100 mg, in patients with overactive bladder (OAB). Methods: Patients received mirabegron 50 mg once daily for 52 weeks. If efficacy was insufficient at week 8, the dose could be increased to 100 mg. Safety was evaluated based on vital signs, adverse events (AEs), laboratory findings, electrocardiogram and post-void residual volume. Treatment efficacy was assessed with a 3-day micturition diary and the King's Health Questionnaire (KHQ). Results: Two hundred and four patients were enrolled; mirabegron dose was maintained at 50mg in 153 patients and increased to 100 mg in 50 patients. Mirabegron was well tolerated at both doses. Incidences of AEs and treatment-related AEs were 91.4% and 33.6% in patients on 50 mg, and 100% and 30.0% in patients on 100 mg, respectively. Time course changes in systolic or diastolic blood pressure and pulse rate were not considered clinically significant. At the end of treatment (EOT), patients on 50mg and 100 mg showed improvement in frequency and urgency. Improvements from baseline to EOT in quality of life scores were observed for all KHQ domains. Conclusions: There were no safety or tolerability concerns associated with mirabegron 50mg (with an optional dose increase to 100 mg) over 52 weeks. Improvement in micturition variables was maintained with mirabegron 50mg from weeks 8 to 52.