We evaluated the effects of long-term maintenance therapy with oral prazosin (6-20 mg, mean 14 +/- 2 mg/d) in 14 patients with congestive heart failure. The patients were followed for 6 +/- 1 months. Eleven of the fourteen patients reported subjective improvement. Two patients required increased diuretics because of gain in body weight. Systolic blood pressure showed a slight but sustained decrease suggesting persistent vasodilator effect. Reductions in echocardiographic left ventricular end-diastolic (6.35 +/- 0.25-5.88 +/- 0.25 cm, p less than 0.05) and end-systolic (5.16 +/- 0.35-4.73 +/- 0.28 cm, p less than 0.05) diameters were observed at 2 months. However, the cardiothoracic ratio on chest x ray was unaltered. Maximum exercise tolerance time increased in eight patients (57%) during prazosin therapy. Improvement in exercise tolerance time was observed in patients with most marked clinical improvement, suggesting presence of cardiac reserve. Two patients died suddenly after reporting subjective improvement. This study shows sustained clinical improvement in most patients with heart failure treated with oral prazosin.