A prospective, randomized efficacy study of low-dose enalapril was undertaken in 38 outpatients (24 men, 14 women; mean age 58 [41-69] years) with chronic heart failure (NYHA functional class III-IV). All patients were pretreated with digitalis and diuretics, some also with conventional vasodilators. 19 patients (group E) received in addition to their previous medication, 5 mg enalapril daily, while the other 19 (group K) continued with their previous therapy. Three months later, 15 patients in group E improved by at least one NYHA functional class and none died (P less than 0.02). Four patients in group K died and only one patient improved by one class. After three months, left ventricular ejection fraction was significantly higher (P less than 0.0001) in group E (39 +/- 19%) compared to group K (30 +/- 14%). In group E, plasma aldosterone concentration decreased significantly (P less than 0.0001) by 33.4 +/- 6.5 ng/dl, while in group K no significant change occurred (delta 1.1 +/- 1.2 ng/dl). Thus, low-dose enalapril in addition to conventional therapy may improve the clinical status of patients in severe chronic heart failure. This improvement is associated with an increase in left ventricular ejection fraction and reduction in secondary hyperaldosteronism.