OBJECTIVE To compare the efficacy and safety of captopril twice a day with quinapril once a day for the treatment of mild to moderate congestive heart failure in a multicentre, double-blind, randomly allocated, parallel-group, 12-week study. METHODS Exercise capacity, M-mode echocardiographic left ventricular measurements and clinical signs and symptoms were evaluated. After a 2-week, single-blind, placebo washout period, the patients were randomly allocated to double-blind treatment with either captopril twice a day or quinapril once a day; the initial doses of 12.5 and 5 mg, respectively, were increased to 25 and 10 mg, respectively, in the first month, and 50 and 20 mg in the second and third months, as required to achieve adequate blood pressure control. Baseline diuretic and/or digitalis therapy was allowed during the study, so that the patients were maintained in a stable condition. RESULTS Data from 70 captopril- and 76 quinapril-treated patients were analysed. Both angiotensin converting enzyme inhibitors displayed comparable efficacy. Both drugs were well tolerated, with a low drop-out rate (11.4% for captopril and 10.5% for quinapril). CONCLUSIONS Treatment with 20 mg quinapril once a day was as effective and safe for the treatment of congestive heart failure as 100 mg captopril administered twice a day.