This proposal describes a double-blind, placebo-controlled randomized clinical trial of the effect of saw palmetto extract on symptoms, objective parameters of disease severity, and quality of life in men with moderate-to-severe benign prostatic hyperplasia. BPH, one of the most common morbid medical conditions in middle-aged and elderly men, is generally treated with alpha-adrenergic blocking agents, finasteride, surgical interventions, or no specific therapy ("watchful waiting"). In the past several years, however, many patients have begun to self-medicate with an extract of the saw palmetto plant (Serenoa repens), a medicinal herb grown in the southeastern United States. Saw palmetto has become the fifth leading medicinal herb consumed in the U.S. and is considered first-line therapy for BPH in several Western European countries. Several small studies suggest that saw palmetto may have clinical benefit, but the methodologic quality of most prior studies has been poor. Very few side effects of the herb have been observed, but few studies have been conducted for more than three months. We propose to conduct a high-quality clinical trial of saw palmetto, with careful attention to the methodologic deficiencies of prior studies. After a single-blind placebo run-in period, 224 patients with mode moderate-to-severe BPH (American Urological Association Symptom Index score greater than or equal to 8) and objective measurement of urinary obstruction, will be randomized to receive either 160mg BID of the herbal extract or an identical placebo. Patients will discontinue any other medical therapy for BPH prior to enrollment and all participants will undergo a trans-rectal ultrasound examination at baseline and closeout. Participants in the trial will be seen at 3-month intervals for a total one-year follow-up. Outcome measurements include changes in the AUASI score (the primary outcome measurement), the peak urinary flow rate, the post-void residual urine volume, the BPH Impact Index, the Olmstead County Study Questionnaire of BPH-specific symptoms and quality of life, and the Short-Form 36 (a generic health status instrument). Numerous laboratory parameters will measured at intervals throughout the trial and symptomatic side effects will be assessed.