In a double-blind study, 116 patients (mean age, 57.6 years) with mild-to-moderate chronic congestive heart failure who were in sinus rhythm were randomly assigned to receive 25 mg of captopril twice daily (up to 50 mg twice daily, if needed) plus hydrochlorothiazide (HCTZ) (group 1) or 0.1 mg of digoxin twice daily plus HCTZ (group 2) for 12 months. During a 3- to 4-week pretreatment stabilization period, group 1 received a mean of 37.7 mg of HCTZ daily and group 2 received 34.9 mg daily. After 6 weeks and 12 months of treatment, improvement was noted in both treatment groups on five measures of cardiac function: exercise tolerance, left ventricular end-diastolic diameter (LVEDD), ejection fraction, blood pressure, and heart rate. At 12 months, significantly greater improvement was noted in group 1 than in group 2 in exercise tolerance (from 329 seconds at baseline to 445 seconds at 12 months in group 1 and from 353 to 427 seconds in group 2; P < 0.05); LVEDD (from 60.5 to 56.5 mm in group 1 and from 60.3 to 57.9 mm in group 2; P < 0.05); and blood pressure (from 103.5 to 95.6 mm Hg in group 1 and from 101.9 to 97.0 mm Hg in group 2; P < 0.03). Clinical severity (New York Heart Association class) improved in both groups; 52% of the patients in group 1 and 41% in group 2 dropped an average of one functional class (P < 0.01). The results indicate that captopril combined with a diuretic is an effective initial treatment for patients with mild-to-moderate congestive heart failure.