Introduction：To evaluate the efficacy and safety of once daily phosphodiesterase type 5 inhibitor(PDE5i) tadalafil as an add-on treatment for men with lower urinary tract symptoms suggestive of benign prostatic hyperplasia（BPH/LUTS）refractory to an alpha 1-blocker（a1-blocker）. Materials and Methods：This study enrolled men aged ＞50 years with BPH/LUTS and erectile dysfunction(ED) that were refractory to ＞3 months of a1-blocker. We defined “refractory” as an international prostate symptoms score (IPSS) of ＞8 and a maximum flow rate (Qmax) ＜15 despite in no space take of an a1-blocker. Patients with contraindications to phosphodiesterase-5 inhibitors；those with symptoms of other diseases that were difficult to differentiate from BPH/LUTS；and those with postvoid residual of ＞100 ml were excluded. Eligible patients received added-on treatment with tadalafil 5 mg/day for 12 weeks. Results：Tadalafil add-on treatment significantly improved the IPSS parameters in terms of total score（P＜0.001）, subscore for storage（P＝0.001）, subscore for voiding（P＜0.001）, and quality of life（P＝0.002）. The 3-day frequency volume chart showed significant improvements in mean number of daily nocturnal micturations（P＝0.002）, nocturnal polyuria index（P＝0.001）, and hours of undisturbed sleep（P＝0.006）. Five patients dropped out because of an adverse event：two with a headache, two with dizziness and one with dyspepsia. There were no serious adverse events. Conclusion：Tadalafil add-on treatment was effective for patients with BPH/LUTS and ED that were resistant to a1-blockers
