The safety and efficacy of once-daily terazosin hydrochloride administered concomitantly with once-daily atenolol for the treatment of essential hypertension were evaluated in this double-blind, multiclinic, placebo-controlled study. After each patient received 50 mg of atenolol daily for eight weeks, patients with a supine diastolic blood pressure (DBP) of 95 to 110 mm Hg and whose supine DBP had decreased at least 5 mm Hg were randomized to receive either terazosin (plus atenolol) or placebo (plus atenolol) for ten weeks. Patients assigned to the terazosin hydrochloride treatment group received increasing dosages (1,2,5, and 10 mg daily) [corrected] of terazosin at two-week intervals until the maximum dose was reached or until the supine DBP was decreased to less than 90 mm Hg. Terazosin-treated patients (n = 43) had significant mean decreases from the baseline in supine BP (systolic/diastolic = -8.8/-8.5 mm Hg) and standing BP (-10.9/-9.5 mm Hg), whereas the decreases in BP in the placebo-treated patients (n = 49; supine, -2.3/-2.6 mm Hg; standing, -1.4/-1.3 mm Hg) were not significant. When terazosin and placebo were compared, the differences in BP were significant. Terazosin-treated patients had significantly greater decreases in mean percent change of total cholesterol (-4.8%) and low-density lipoprotein plus very-low-density lipoprotein cholesterol (-6.3%) levels, compared with the placebo-treated patients (+0.6% and +1.1%, respectively). Concomitant administration of terazosin and atenolol to patients with essential hypertension was found to be safe and efficacious.