The pharmacology and pharmacokinetics of finasteride are reviewed, and finasteride's clinical efficacy, adverse effects, and dosage in patients with benign prostatic hyperplasia (BPH) are described. Finasteride is a member of a new class of medications, the azasteroids, that have antiandrogenic activity limited to certain tissues, including the prostate gland. Finasteride inhibits the activity of 5 alpha-reductase, the enzyme necessary for converting testosterone to dihydrotestosterone in the prostate and other tissues. BPH is androgen dependent, and dihydrotestosterone is necessary for the hyperplasia to occur. Finasteride is well absorbed after oral administration; peak plasma concentration is reached in as little as one hour. Finasteride is approximately 90% bound to plasma proteins, is widely distributed, and is extensively metabolized by the liver. In clinical studies, finasteride has been effective in decreasing prostatic volume, increasing urinary flow rate, and decreasing the obstructive and irritative symptoms of BPH. Because of the heterogeneity of BPH, however, treatment is successful in only one third to one half of patients. Finasteride's adverse effects include decreased libido, ejaculatory disorders, and effects on prostate-specific antigen. The recommended dosage of finasteride is 5 mg/day orally. Therapy should be continued for at least six months before the clinical response is evaluated. Finasteride provides a pharmacologic alternative to surgery in patients with BPH.