The efficacy and safety of treatment with finasteride 5 mg daily for 24 months was assessed in this multicentre double blind placebo-controlled study including 707 patients with moderately symptomatic benign prostatic hyperplasia. Efficacy parameters were changes in voiding- and bladder storage symptoms assessed by a validated symptom score, changes in maximum urinary flow rate and changes in the prostate volume. In the finasteride patients, symptom score improved during the whole study with a significant difference between active treatment and placebo after 24 months (p < 0.01). Maximum flow rate increased in finasteride treated patients resulting in a difference between these and the placebo treated patients of 1.8 ml/s after 24 months (p < 0.01). Prostate volume was reduced by 19% in the finasteride treated patients versus an increase of 12% in the placebo treated patient group (p < 0.01). Finasteride was well tolerated. Patients receiving placebo progressed in symptoms after 16 months. Finasteride can halt the natural progression of moderately symptomatic BPH over a 24 month period.