ObjectivesTo investigate urodynamic efficacy and safety of mirabegron add-on treatment with tamsulosin for Japanese male patients with overactive bladder (OAB). MethodsA prospective study was conducted in 26 consecutive male patients with OAB who had been taking tamsulosin. OAB was diagnosed by overactive bladder symptom score (OABSS). Before and 8weeks after mirabegron add-on treatment with preceding tamsulosin, we assessed OABSS, International Prostate Symptom Score (IPSS), free uroflowmetry (UFM), filling cystometry and pressure-flow study (PFS). ResultsMean age and prostate volume of the study patients were 757years and 32 +/- 19mL, respectively. Mirabegron significantly improved OABSS (from 8.5 +/- 2.3 to 4.7 +/- 2.5, P<0.001). On free UFM, mirabegron significantly increased voided volume (from 135 +/- 47 to 182 +/- 102mL, P=0.01), maximum (from 10.7 +/- 3.7 to 13.5 +/- 6.4mL/sec, P<0.01) and average flow rate (from 5.5 +/- 1.9 to 7.1 +/- 3.3mL/sec, P<0.01), while postvoid residual urine volume did not change significantly (from 47 +/- 38 to 63 +/- 61mL, P=0.23). Before mirabegron, 24 patients (92%) had detrusor overactivity (DO). After mirabegron add-on, maximum cystometric capacity significantly increased from 170 +/- 98 to 212 +/- 95mL (P=0.01) and DO disappeared in six patients (25%). In the other 18 patients with persistent DO, amplitude of involuntary contraction decreased and bladder volume at first involuntary contraction increased with statistical significance. On PFS, detrusor pressure at maximum flow rate (from 79 +/- 31 to 68 +/- 19cmH(2)O, P=0.10) or bladder contractility index (from 126 +/- 39 to 120 +/- 27, P=0.45) did not change significantly. Conclusions: Mirabegron add-on treatment with tamsulosin has efficacy and safety because it improves storage symptom without impairment of bladder contractility during voiding in male patients with OAB.