We evaluated the clinical efficacy and safety of administering udenafil (5-[2-propyloxy-5-(1-methyl-2-pyrollidinylethylamidosulphonyl)phenyl]-1-methyl-3-propyl-1,6-dihydro-7H-pyrazolo(4,3-d)-pyrimidin-7-one) in patients with comorbid benign prostatic hyperplasia (BPH) and erectile dysfunction (ED). One hundred and twenty patients who had been undergoing stable alpha-blocker therapy for BPH were enrolled in this trial and they were administered 100 mg udenafil for 8 weeks. Changes in blood pressure (BP), heart rate (HR), the international prostatic symptom score (IPSS) and the international index of ED (IIEF-5) were evaluated every 4 weeks. At end point, there was no significant change in BP and HR, whereas the lower urinary tract symptoms (LUTS) and ED improved significantly compared with baseline (IPSS 14.3-11.5, IIEF-5 11.95-18.32, P<0.05). Most patients were tolerant to the treatment and there was no evidence of additional side effects related to coadministration. The coadministration of udenafil and an alpha-blocker in patients with comorbid BPH and ED was safe and gave significant improvements in both LUTS and ED.