Ninety-four patients on digitalis treatment for chronic congestive heart failure (NYHA class II-III) were enrolled for a 12 month trial in a random, double-blind, placebo-controlled study. After a placebo run-in period, patients were assigned to placebo or captopril 25 mg t.i.d. Digitalis was continued while diuretics were withdrawn. Clinical status, exercise capacity, cardiac dimensions and performance were evaluated with a full physical examination, 12 lead ECG, chest X-ray, 24 hour Holter monitoring, bicycle effort capacity, M-mode echocardiography and radionuclide ventriculography at 1, 2 and 3 weeks and 1, 2, 3, 6 and 12 months. There were no significant differences in the trend of survival curves after six months follow-up between the captopril or placebo treatment groups. Patients treated with captopril, without the addition of diuretics, had an improvement in NYHA class (P less than 0.01), an increase in exercise capacity (P less than 0.025), a decrease in cardiothoracic ratio (P less than 0.025) and an increase of echocardiographic left ventricular contractility (P less than 0.005). Only four patients treated with captopril were withdrawn from the follow-up for allergic side effects. Preliminary results at 6 months prove that captopril, compared to placebo, is useful in mild to moderate heart failure.