Vasodilators may be required when signs of cardiac failure persist, despite adequate digitalo-diuretic therapy. Prazosin is a post-synaptic alpha-blocker which acts on both cardiac preload and afterload. For this reason, it has been widely used in the treatment of cardiac failure. We used prazosin in an open uncontrolled trial in 17 patients with an average of 59 years, in whom Stage III or IV cardiac failure persisted despite digitalis and diuretic therapy. Haemodynamic data obtained with a Swan Ganz catheter was used to judge the effectiveness of an initial dose of prazosin and long-term results were assessed by repeat studies after 6 and 10 weeks of continuous therapy. After the first, we observed a marked fall in pulmonary capillary (15.5% 7.4 vs 22.9% 8.8 mm Hg, p less than 0.01) and mean pulmonary artery pressures (23.8% 9.2 vs 34.2 +/- 10.6 mm Hg, p less than 0.001). Systemic vascular resistances were also significantly reduced (1 370 +/- 406 vs 1 983 +/- 464 dynes.s.cm-5, p less than 0.001). There was a moderate fall in mean systemic blood pressure (80.8% 10.6 vs 95.6 +/- 129 mm Hg, p less than 0.001). Cardiac index increased significantly (2.7 +/- 0.68 vs 2.13% 0.56 1/min/m2, p less than 0.01). The heart rate was constant. The maintenance dose was 5 mg three times daily in 9 cases, and 10 mg three times daily in the other 8 cases. The medium term results were assessed in 14 patients as 2 patients died and 1 stopped treatment for undetermined reasons. The symptomatic improvement was marked (class 2.5 +/- 0.76 vs 3.64 +/- 0.49, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)