The safety, efficacy and the effect on the plasma total cholesterol of once-daily terazosin hydrochloride administered either as monotherapy or in combination with other antihypertensive therapy were evaluated in patients with mild-moderate uncontrolled essential hypertension in this U.K., open, multicentre hospital, 3-month study. Patients initially received 1 mg of terazosin as monotherapy or in addition to their current antihypertensive therapy followed by dose titration, if necessary, to a maximum of 10 mg over the first 6 weeks depending upon blood pressure control. Patients then continued treatment for a further 6 weeks on their optimum dosage. There were highly significant mean reductions in systolic and diastolic blood pressures at the end of 12 weeks for 439 patients of 18.5 and 14.0 mmHg (P < 0.001), respectively, and were similar to the reductions shown when subgrouping the patients into either those 197 patients who were treated with terazosin as monotherapy (17.6/13.7 mmHg, P < 0.001) or those 242 patients treated with terazosin in combination with other antihypertensive agents (19.3/14.2 mmHg, P < 0.001). In addition, in a sub-group of 132 patients who had their mean total cholesterol measured at the end of 12 weeks' treatment, there was a significant reduction of -0.4 mmol/l (P < 0.01). A similar significant mean reduction for total cholesterol was also shown when this subgroup was divided into those 50 patients who were treated with terazosin as monotherapy (-0.56 mmol/l, P < 0.01), but not for those 82 patients treated with terazosin in combination with other antihypertensive agents (-0.3 mmol/l, P > 0.05 < 0.1). No serious toxicity or safety problems were observed. Once daily administration of terazosin, either as monotherapy or in combination with other antihypertensive agents, proved to be safe and effective in reducing the blood pressure of patients with uncontrolled mild-moderate essential hypertension and favourably reduced the total plasma cholesterol level.