Controlled clinical studies have demonstrated that blockade of alpha 1-adrenergic receptors relaxes prostatic muscle tone and decreases the symptoms of benign prostatic hyperplasia (BPH). Doxazosin, a once-daily quinazoline derivative and postsynaptic alpha 1-adrenoceptor antagonist, proven as treatment for hypertension, was evaluated in the treatment of BPH in dosages of 1-16 mg. 456 BPH patients (287 doxazosin-treated and 169 placebo-treated) were evaluated for efficacy and safety in five double-blind, placebo-controlled clinical studies. Doxazosin treatment resulted in improvements in both urodynamic and symptomatic parameters associated with BPH. Efficacy was only assessed in 1, 2 and 4 mg. Adverse experiences were reported in 127 (44.3%) of the patients treated with doxazosin and in 49 (29%) of the patients treated with placebo. Fifteen (5.2%) doxazosin patients and 4 (2.4%) placebo patients withdrew from the studies due to adverse effects. Results from these five clinical trials demonstrate doxazosin is effective and safe and well tolerated in both normotensive and hypertensive patients with BPH.