Lisinopril 5–20 mg once daily was compared with digoxin 0.125–0.375 mg once daily in a double-blind, randomized, parallel-group study involving 217 patients with mild-to-moderate heart failure (New York Heart Association [NYHA] grades II–III) who were maintained on optimized diuretic therapy. After 6 weeks of treatment, digoxin and lisinopril had increased exercise duration by 18 seconds (p = 0.015) and 32 seconds (p = 0.0007), respectively, versus the baseline run-in period. The difference between treatments was not statistically significant (p = 0.1343). After 12 weeks, digoxin and lisinopril had increased exercise duration by 29 seconds and 51 seconds, respectively. The effect of digoxin compared with the baseline value was not significant but that for lisinopril was (p = 0.0027). The difference between treatments approached statistical significance (p = 0.0813). There was no difference between lisinopril and digoxin with regard to their effects on the frequency of ventricular ectopic counts, couplets, or nonsustained ventricular tachycardia. Blood pressures were not significantly different between treatments, although both systolic and diastolic blood pressure were consistently lower in the lisinopril group throughout randomized treatment. The proportions of patients demonstrating an improvement in NYHA grading were similar for both lisinopril and digoxin. Both treatments had similar effects on the symptoms of heart failure. Both drugs appeared to be equally well tolerated with a similar frequency of adverse events reported for both drugs (30% for lisinopril vs 29% for digoxin). Withdrawals from treatment were of a similar frequency for both treatments. It is concluded that lisinopril may be a useful alternative to digitalis in patients with heart failure who remain symptomatic on diuretic therapy.