One hundred and seven patients with chronic heart failure (NYHA class II to IV) stabilized on digitalis and/or diuretics, recruited from 11 centres were randomized into a double-blind, placebo-controlled study to assess the effect of 12 weeks of cilazapril therapy on exercise tolerance and clinical status. Thirty-five patients were randomized to placebo and 72 to cilazapril at a starting dose of 1 mg daily; titration to cilazapril 2.5 mg at week 4 and 5 mg at week 8 (or matching placebo) was carried out in patients who did not improve clinically. Demographic characteristics, including exercise test duration increased from 402 s (+/- 17 SEM) at baseline to 462 s (+/- 19 SEM) at week 12 for the cilazapril group (+15%) and from 405 s (+/- 23 SEM) at baseline to 408 s (+/- 30 SEM) at week 12 in patients on placebo (+1%) (P < 0.001). In the placebo group, patients able to exercise for more than 6 min at baseline showed an increase in exercise duration at week 12 while those able to exercise for up to 6 min at baseline showed a decrease (P = ns). In contrast, cilazapril-treated patients showed an increase in exercise tolerance regardless of baseline exercise test duration; patients with the most impaired exercise tolerance at baseline showed a greater improvement than patients with mildly impaired baseline exercise tolerance (P < 0.05 vs placebo). NYHA class improved by at least one grade in 51% of the cilazapril group vs 32% in the placebo group (P = ns). At the end of the trial, 15% of the patients were non-responders on cilazapril vs 41% on placebo (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)