A group of 205 patients with mild to moderate essential uncomplicated hypertension was chosen from 3,183 hypertensive patients referred to a hypertension clinic for the first time, and was asked to participate in a six-month double-blind parallel trial. A single physician was in charge of all the patients. After a two-week single-blind placebo period, the patients were randomly assigned to regimens of either enalapril (20 mg per day) or placebo. Both groups were then followed up every two weeks, and increasing doses of hydrochlorothiazide (25 and 50 mg), oxprenolol (160 and 320 mg), and dihydralazine (50 and 100 mg) were added until the diastolic blood pressure was lower than 90 mm Hg. After a six-month follow-up, the enalapril group showed lower systolic and diastolic blood pressures than the control group (129/82 +/- 12/6 mg Hg versus 135/86 +/- 10/5 mm Hg; p less than 0.001). The number of daily tablets of active drugs was 2.7 +/- 1.8 in the enalapril group and 4.4 +/- 2.4 in the control group (p less than 0.01). The mean plasma potassium level was 4.16 +/- 0.4 mmol/liter in the enalapril group versus 3.92 +/- 0.4 mmol/liter in the control group (p less than 0.001), despite more frequent use of amiloride (p less than 0.001). This difference is explained by the lower dose of hydrochlorothiazide used in the enalapril group by comparison to the control group, and a lower excretion of urinary aldosterone in the enalapril group than in the control group (11.6 +/- 7.4 versus 19.8 +/- 11.8 micrograms per 24 hours, p less than 0.001). Drug withdrawal was necessary in eight patients in the enalapril group and in 16 patients in the control group (p less than 0.05). These results show that first-step treatment of mild to moderate uncomplicated essential hypertension with enalapril permits better blood pressure control than the standard treatment, requires fewer tablets to be taken daily, and involves a smaller risk of hypokalemia.