The safety and efficacy of the selective alpha 1-blocking agent doxazosin 4 mg once daily in the symptomatic treatment of benign prostatic hyperplasia were evaluated in a randomized, double-blind and placebo-controlled 9-week study of 100 patients. By patients' overall assessment of voiding difficulties, 79% in the doxazosin group (DG) and 44% in the placebo group (PG) reported improvement (p = 0.001). In the DG, improvement was noted in 63% of obstructive symptoms compared to 32% in the PG (p = 0.015), whereas improvement was noted in 76% and 45%, respectively, of irritative symptoms (p = 0.12). Daytime frequency was reduced by 1.5 in the DG and increased by 0.3 in the PG (p = 0.001), and nocturia was reduced by 1.1 and 1.0, respectively (p = 0.12). Maximum urinary flow rate was improved by 1.5 ml/s in the DG, while it deteriorated by 0.3 ml/s in the PG (p = 0.11). Considering postvoid residual urine volume, cystometry variables (first sensation and bladder capacity) and adverse events there was no difference between the two groups. In conclusion, doxazosin 4 mg once daily is safe and effective in relieving symptoms in patients with BPH.