After two weeks of a wash-out run-in period with placebo, 131 patients with congestive heart failure (New York Heart Association [NYHA] class II to III) and left ventricular ejection fraction </=40% were randomly assigned to a treatment period of 4 weeks with 10 mg quinapril once daily or 12.5 mg captopril twice daily. At the end of this period, doses were titrated to 20 mg quinapril once daily or 25 mg captopril twice daily on the basis of physician judgment if there were no major adverse reactions and if blood pressure was not below 110/70 mm Hg. Clinical symptoms of heart failure were significantly relieved by both drugs at the end of a 12-week treatment period. At the beginning of the study, 23 (35%) of the 65 patients taking quinapril and 27 (41%) of the 66 patients taking captopril were in NYHA functional class III, whereas, at the end of the trial, only 4 (6%) of the patients in the quinapril group and 14 (22%; p < 0.05 versus quinapril) patients in the captopril group were classified as NYHA class III. Both drugs had a positive effect on echocardiographic parameters. There was a statistically significant increase in exercise duration in both treatment groups (quinapril, 6.2 +/- 1.8 versus 7.8 +/- 1.9 minutes, p < 0.001; captopril, 5.9 +/- 1.9 versus 7.1 +/- 2.3 minutes, p < 0.001). One patient in the quinapril group died suddenly during the study and two patients in the captopril group dropped out of the study due to persistent dry cough. No patient in the quinapril group reported side effects. Three patients in the captopril group suffered from moderate dry cough, one from taste-blindness, and another from unstable angina. The safety of the tested drugs was confirmed by laboratory tests. Quinapril was as effective as captopril in reducing signs and symptoms of heart failure and in improving the left ventricular function and the exercise capacity with few side effects.