We investigated the add-on effect of dutasteride (0.5 mg once a day) on lower urinary tract symptoms (LUTS), prostate volume (PV), and serum prostate specific antigen (PSA) and testosterone level in 72 patients with benign prostatic hyperplasia (BPH) who had been treated with alpha-blocker monotherapy. Inclusion criteria were men with BPH who had PV ≧30 ml and international prostate symptom score (IPSS) ≧8 or quality of life (QOL) index ≧3 under alpha-blocker monotherapy for more than 3 months. At the baseline, 12 and 24 weeks after dutasteride add-on, we assessed IPSS, overactive bladder symptom score (OABSS), PV, serum PSA and testosterone. Among 47 patients (65%) with OAB diagnosed by OABSS, responders were defined as those with urgency score of OABSS ＜2 or total score of OABSS ＜3. At the 24th week, dutasteride significantly improved IPSS (-4.2) and OABSS (-1.9) and reduced PV (-29%) compared with the baseline. Furthermore, dutasteride significantly decreased serum PSA (-45%) and increased testosterone (36%). Among OAB patients, dutasteride significantly improved urgency and urgency incontinence but not nocturia. Responders had lower OABSS, urgency incontinence score and serum testosterone at the baseline than non-responders. In conclusion, dutasteride add-on therapy is beneficial in patients with BPH who do not show enough improvement with alpha-blocker monotherapy.