In a randomized, double-blind, placebo-controlled, multicenter trial, the efficacy and safety of terazosin in combination with other antihypertensive agents were assessed using patients with inadequately controlled essential hypertension (supine diastolic blood pressure 95 mm Hg or greater). Of 138 evaluable patients, the terazosin-treated group (n = 84) achieved a significant (p less than 0.05) mean reduction of 7.3 mm Hg in supine diastolic blood pressure when compared with the placebo-treated group (n = 54) who achieved a reduction of 0.6 mm Hg. Analysis of patients by background therapeutic categories (beta blocker, beta blocker plus diuretic, or "other") revealed that terazosin caused a significantly greater decrease in supine diastolic blood pressure than did placebo when added to the combination of beta blocker plus diuretic (decrease of 7.2 mm Hg for the terazosin group versus a decrease of 1.5 mm Hg for the placebo group) and to other antihypertensive drugs (decrease of 7.9 mm Hg for the terazosin group versus a decrease of 1.0 mm Hg for the placebo group). Significant differences between treatment groups were also observed for supine systolic and for standing systolic and diastolic blood pressure variables in select subgroups. The addition of terazosin did not cause significant changes in physical examinations or electrocardiograms. Changes in pulse rates and body weight did not differ significantly between treatment groups for any background therapeutic category. The overall incidence of adverse experiences was only somewhat greater for terazosin-treated than placebo-treated patients. These results suggest that terazosin is safe and effective when administered in combination with other antihypertensive drugs.