367 patients with mild-moderate hypertension were included in a multicentre study for the purpose of examining the antihypertensive effect of six weeks of treatment with the ACE-inhibitor lisinopril 10 and 20 mg once daily. Both low-dose and high-dose lisinopril significantly reduced sitting and standing blood pressure values. The fall in blood pressure in the sitting position was slightly but significantly greater among the high-dose group compared with the low-dose group (a 3 mm Hg fall difference in systolic values and a 1 mm Hg fall in diastolic values). No such differences were found in the standing position. Heart rate remained unchanged during lisinopril treatment. Episodes of possible first dose hypotension were reported in six patients. Approximately 90% of the patients in both groups were classified as responders according to defined criteria. The frequency of side-effects was low, and was equal in both treatment groups. An evaluation of reduction in blood pressure, and of response rate and side-effects suggests that an initial dose of 10 mg lisinopril once daily is sufficient, and that this dosage will control blood pressure in the majority of patients.