The effectiveness of enalapril 10-40 mg/day as first choice treatment of mild (90-104 mmHg, n = 37), moderate (105-114 mmHg, n = 21), or severe (115-130 mmHg excluding accelerated hypertension, n = 16) essential hypertension was studied in an open multicentre trial. Enalapril alone controlled the hypertension (diastolic blood pressure 90 mmHg or less) in 25 patients (34%). Of these, 20 had mild and 5 had moderate hypertension. The remaining patients required either enalapril plus hydrochlorothiazide 12.5 or 25 mg/day (n = 30), or a third drug of the physician's choice (n = 9). A relationship was present between baseline blood pressure and the number of drugs required to achieve blood pressure control. Plasma creatinine increased beyond the limits of laboratory error in 3 patients, and from 100-108 mumol/l (p less than 0.05) on enalapril alone in a subgroup of patients who ultimately required a diuretic. Enalapril was well tolerated; 60 (73%) had no drug related side effects during active treatment. Tiredness (n = 5), headache (n = 4), dizziness (n = 4) and palpitations (n = 3) were the most frequent side effects. Cough was a feature in 3 patients and 1 patient had a rash. This study suggests that enalapril is an effective and well tolerated anti-hypertensive agent in mild, moderate or severe hypertension, but that caution may be required in patients with impaired renal function.