Thirty adults with essential hypertension (systolic BP greater than 150 mmHg or diastolic BP greater than 100 mmHg) were treated with 5-20 mg of enalapril to study its anti-hypertensive efficacy and safety. Ten patients had mild hypertension (diastolic BP greater than 90 mmHg and less than 105 mmHg), 10 had moderate hypertension (diastolic BP greater than 105 mmHg and less than 115 mmHg) and 6 had severe hypertension (diastolic BP greater than 115 mmHg). Of the 20 patients who completed the trial, 9 (45%) showed optimum reduction of BP to less than 130/90 mmHg) and a further 7 (35%) showed significant reduction. The mean fall in systolic BP was 32.5 mmHg (0-80 mmHg) and in diastolic BP was 18.5 mmHg (0-50 mmHg). The peak fall in BP was achieved in 3.5 weeks (1-6 weeks) with a mean dose of 7.2 mg of enalapril daily (5-15 mg/day). Mild side effects not needing drug withdrawal were seen in 8/20 patients (40%). Monotherapy with enalapril appears to be effective and safe as step one therapy for patients with essential hypertension.