Twelve patients with severe chronic congestive heart failure (CHF) received prazosin in a dose of 4 to 20 mg daily for a period of 2 months, in addition to cardiac glycoside and diuretic drugs. After this treatment left ventricular end-diastolic diameter decreased from 5.7 +/- 0.4 to 5.4 +/- 0.4 cm (p less than 0.001), left ventricular end-systolic diameter decreased from 4.3 +/- 0.5 to 4 +/- 0.5 (p less than 0.001), left ventricular stroke excursion increased from 1.03 +/- 0.11 to 1.27 +/- 0.15 cm (p less than 0.01), the ejection fraction increased from 24.9 +/- 2.1 to 32.2 +/- 2.8 (p less than 0.001) and the mean velocity of circumferential fiber shortening increased from 0.68 +/- 0.06 to 0.79 +/- 0.08 circumferences/s (p = 0.06). Prazosin treatment clearly relieved dyspnea and fatigue. Three patients improved from Class IV to Class III, four patients improved from Class IV to Class II and 3 patients from Class III to Class II.