To evaluate the influence of the angiotensin-converting-enzyme inhibitor enalapril (2.5 to 40 mg per day) on the prognosis of severe congestive heart failure (New York Heart Association [NYHA] functional class IV), we randomly assigned 253 patients in a double-blind study to receive either placebo (n = 126) or enalapril (n = 127). Conventional treatment for heart failure, including the use of other vasodilators, was continued in both groups. Follow-up averaged 188 days (range, 1 day to 20 months). The crude mortality at the end of six months (primary end point) was 26 percent in the enalapril group and 44 percent in the placebo group--a reduction of 40 percent (P = 0.002). Mortality was reduced by 31 percent at one year (P = 0.001). By the end of the study, there had been 68 deaths in the placebo group and 50 in the enalapril group--a reduction of 27 percent (P = 0.003). The entire reduction in total mortality was found to be among patients with progressive heart failure (a reduction of 50 percent), whereas no difference was seen in the incidence of sudden cardiac death. A significant improvement in NYHA classification was observed in the enalapril group, together with a reduction in heart size and a reduced requirement for other medication for heart failure. The overall withdrawal rate was similar in both groups, but hypotension requiring withdrawal occurred in seven patients in the enalapril group and in no patients in the placebo group. After the initial dose of enalapril was reduced to 2.5 mg daily in high-risk patients, this side effect was less frequent. We conclude that the addition of enalapril to conventional therapy in patients with severe congestive heart failure can reduce mortality and improve symptoms. The beneficial effect on mortality is due to a reduction in death from the progression of heart failure.