Catopril, an angiotensin converting enzyme inhibitor, was administrated in 30 hypertensive patients(mild 8, moderate 12, severe 10 cases)for 12 weeks or longer in order to observe the hypertensive effects. Changes in quality of life, side effects, electrocardiogram and left ventricular mass index(LVMI) by 2D-guided M-mode echocardiography were also evaluated before and after captopril. 1) After 12 weeks treatment with 25 to 150mg of captopril alone, blood pressures were lowered markedly in 16(53%), moderatly in 5(17%) and midly in 2(7%), while the addition of 25mg hydrochlorthiazide to captopril in the patients who showed no satisfactory responses the blood pressure were lowered markly in 21(70), moderately in 6(20%) and mildly 3(10%) out of 30 patients studied. The average of blood pressure of the 30 subjects were systolic 180.7+/-20.7mmHg(M+/-SD) and diastolic 113.2+/-12.5 before treatment, which were lowered to 148+/-15.8 and 92.5+/-8.0mmHg respectively after 12 weeks(P<0.005). 2) Heart rates were not changed with captopril and/or hydrochlorothiazide. 3) Quality of life improved markedly in 5(17%) and slightly in 12(40%) out of 30 subjects. 4) Complete blood cell count, urinalysis and serum enzymes followed revealed no significant changes. 5) By electrocardiographic follow-up studies 1 out of 13 subjects with LVH, 1 out of 4 LAH, 1 out of 2 ST-T changes were revealed to improved to normal. 6) In 25 out of 30 cases left ventricular mass indices(LVMI) were above 125g/m2 before treatment, among which 15 cases were followed with satisfactoriness good quality of the echocardiographic recorings and the LVMI was reduced from 169.6+/-40.7 to 141.7+/-40.9g/m2(P<0.01). 7) Undesirable side effects were dry cough 3, skin rash 2, dysgeusia 1 and renal dysfunction 1. 8) Considering the blood pressure lowering effects, life quality changed and side effects together the captopril was considered very useful in 8(27%) and useful in 16(53%) out of 30 subjects. Above results suggest that captopril can be prescribed as an effective initial single agent or with in combinations with thiazide for the treatment of hypertensive of various severities with acceptably low side effects.