OBJECTIVE: To investigate the effect of combination therapy with dutasteride plus tamsulosin compared with each monotherapy on patient-reported health outcomes over 4 years in men with moderate-to-severe lower urinary tract symptoms (LUTS) because of benign prostatic hyperplasia (BPH). METHODS: CombAT was a 4-year international, double-blind, randomised, parallel-group trial in men (n = 4844) with moderate-to-severe symptoms of BPH and at increased risk of disease progression [age > or = 50 years, International Prostate Symptom Score (IPSS) > or = 12, prostate volume > or = 30 cc, serum prostate-specific antigen > or = 1.5 ng/ml to < or = 10 ng/ml and maximum urinary flow rate 5-15 ml/s with minimum voided volume > or = 125 ml]. Subjects were randomised to receive 0.5 mg dutasteride, 0.4 mg tamsulosin or the combination once daily for 4 years. The primary endpoint at 4 years was the time to event and proportion of subjects with acute urinary retention or undergoing BPH-related prostate surgery. Secondary endpoints included the health-outcomes measures, BPH Impact Index (BII), IPSS question 8 (IPSS Q8) and the Patient Perception of Study Medication (PPSM) questionnaire. RESULTS: At 4 years, combination therapy resulted in significantly superior improvements from baseline in BII and IPSS Q8 than either monotherapy; these benefits were observed from 3 months onwards compared with dutasteride and from 9 months (BII) or 12 months (IPSS Q8) onwards compared with tamsulosin. Also at 4 years, the PPSM questionnaire showed that a significantly higher proportion of patients was satisfied with, and would request treatment with, combination therapy compared with either monotherapy. CONCLUSIONS: Combination therapy (dutasteride plus tamsulosin) provides significantly superior improvements in patient-reported quality of life and treatment satisfaction than either monotherapy at 4 years in men with moderate-to-severe BPH symptoms.