The short-term antihypertensive efficacy and safety of terazosin when administered with a diuretic were assessed in three randomized, double-blind, placebo-controlled studies. In all studies, adding terazosin to an established diuretic regimen resulted in significant incremental decreases in blood pressure from baseline to the final visit, with an overall satisfactory response rate of 56 percent versus 29 percent for control subjects. Mean decreases in supine diastolic blood pressure in the terazosin-treated groups ranged from 4.5 to 8.9 mm Hg, compared with mean decreases of 0.4 to 5.8 mm Hg in the placebo-treated groups; these differences generally achieved statistical significance. There were no clinically or statistically significant changes in pulse rates, physical examinations, or electrocardiographic tracings. Results of clinical laboratory tests revealed no evidence of drug-induced toxicity. Patients receiving the terazosin plus diuretic combination had a slight tendency to gain weight. The most common adverse effects reported by this group were dizziness, headache, and asthenia. The results of these studies indicate that combination therapy with terazosin plus a diuretic is both safe and effective for the treatment of hypertension.