In a double-blind, placebo-controlled, randomized, multicenter study, 225 patients with mild to moderate congestive heart failure (CHF) due to arterial hypertension and ischemic heart disease received quinapril, a new non-sulfhydryl angiotensin-converting enzyme (ACE) inhibitor, in addition to maintenance therapy with digitalis and/or diuretics. The patients were randomized into four groups: (a) placebo; (b) quinapril, 5 mg twice daily (b.i.d.); (c) quinapril, 10 mg b.i.d.; and (d) quinapril, 20 mg b.i.d. All four groups were treated for 12 weeks. Patients receiving quinapril showed significant improvement in exercise time and NYHA functional class compared with those receiving placebo. A subset of 26 patients who received neither digitalis nor diuretics showed the same beneficial response after quinapril administration that was shown by patients who received maintenance therapy. After the double-blind trial, 189 patients entered a 1-year, open-label study in which the quinapril dosage could be titrated to a maximum of 40 mg/day (20 mg b.i.d.). The beneficial effect of quinapril was maintained over 12 months even when most patients received lower doses than in the double-blind trial.