We studied the effects of 5 alpha-reductase inhibitor (finasteride) in the treatment of benign prostatic hyperplasia (BPH). This study is a randomized controlled trial. Sixty-two patients were treated with 5 alpha-reductase (finasteride 5 mg/day) and 61 patients (control group) with placebo for one year. Prostatic volume, maximal urine flow rate, AUA symptom scoring, residual urine volume and prostate-specific antigen (PSA) levels were evaluated at 3, 6, 9 and 12 months. In the first 6 months prostatic volume decreased rapidly (20.5%), in the second 6 months it decreased slowly and reached the maximal rate (23.3%). Maximal urine flow rate increased in the second 6 months. AUA symptom scores decreased first at 3 months and were 4.6 points lower at the end of the 12th month. There were no significant changes in residual volume. The 5 alpha-reductase inhibitor caused a 50% decrease in PSA levels, like in other studies. Because of the prolonged use of the drug, treatment with 5 alpha-reductase inhibitor is not tolerated by many patients and being expensive its future in the pharmacotherapy of BPH is unclear.