The aim of the study was to evaluate efficacy and tolerability of two different fixed combinations of an angiotensin-converting enzyme inhibitor and a diuretic: delapril+indapamide (D+I) and captopril+hydrochlor-othiazide (C+H) administered for 6 months to patients with mild to moderate essential hypertension. In all, 96 centres participated in this randomised, parallel groups, controlled study. A total of 829 patients with uncomplicated mild to moderate hypertension were randomised, and 790 were eligible for the analysis of efficacy (intention to treat). Patients of both sexes, aged 18-75 years, newly diagnosed or untreated during the last month were included in the study if their diastolic blood pressure (DBP) was X95 and p114 mmHg. The starting doses of the drugs were delapril 30 mg+indapamide 1.25 mg tablets o.d. or captopril 50 mg+hydrolchlorothia-zide 15 mg tablets o.d. After a 1-month treatment period, nonresponders (DBP 490 mmHg, or decrease in DBP o10 mmHg) had the daily dose increased to either delapril 30 mg+indapamide 2.5 mg or captopril 50 mg+hydrochlorothiazide 25 mg tablets for a further 5 months. The primary assessment of antihypertensive efficacy was the percentage of patients who responded after a 6-month drug treatment. The responder rates were 72.6% with D+I and 62.9% with C+H (P ¼ 0.004 between treatments) after 60 days of treatment, and 92.6% in the D+I and 85.2% in the C+H (Po0.001 between treatments) at the end of the treatment period. The final value of systolic blood pressure was 134.5 7 13.1 mmHg with D+I and 138.3 7 14.0 mmHg with C+H (Po0.001 between treatments). At the final visit, DBP was 84.577.0 mmHg in the D+I group and 85.578.0 mmHg in the control group (P ¼ 0.017 between treatments). In all, 11 patients in the D+I group and 19 patients in the C+H group were withdrawn from the study because of adverse events. In all, 30 patients (7.6%) with D+I and 32 patients (8.1%) with C+H experienced adverse events. In conclusion, D+I was more effective than C+H in terms of overall reduction in blood pressure and response rate. Greater efficacy was obtained without any increase in adverse effects, since both treatments were equally well tolerated.