Objective: To investigate the effect of dutasteride versus placebo on the symptoms and associated complications of male lower urinary tract symptoms and benign prostatic hyperplasia (BPH) across a range of prostate volumes and BPH symptoms in men evaluated for prostate cancer risk reduction in the 4-year REduction by DUtasteride of prostate Cancer Events (REDUCE) trial. Methods: REDUCE was a multicenter, randomized, double-blind, placebo-controlled study of prostate cancer risk reduction with daily dutasteride 0.5 mg or placebo. Eligible men were aged 50-75 years, with a prostate-specific antigen level of 2.5-10 ng/mL and a prostate volume of ≤80 cm3. The prespecified and post hoc analyses were performed on the incidence of acute urinary retention, BPH-related surgery, and urinary tract infections, as well as on changes in prostate volume, International Prostate Symptom Score, BPH Impact Index, and maximal urinary flow rate (Qmax). Results: A total of 8122 men were included in the efficacy population. During the 4-year study, the International Prostate Symptom Score increased in placebo-treated patients, while dutasteride-treated patients had a stabilized or decreased International Prostate Symptom Score and improved BPH Impact Index and quality of life due to urinary symptom scores across all prostate volume quintiles (including prostate glands smaller than those studied in previous dutasteride trials). 48 months, the incidence of acute urinary retention or BPH-related surgery was significantly less in the dutasteride group (2.5%) than in the placebo group (9%) overall (P <.001) and in each baseline prostate volume quintile (P <.01). Conclusion: During the 4-year study, dutasteride was associated with a decreased risk of BPH progression in men with mild-to-moderate symptoms and normal or enlarged prostates.