To compare the value of the New York Heart Association (NYHA) classification and measurement of peak oxygen consumption (VO2) in the assessment of functional status and prognosis in patients with mild to moderate chronic congestive heart failure (CHF), 94 patients with clinically stable NYHA class II and III CHF were prospectively studied. In all patients, left ventricular ejection fraction was less than or equal to 40% (mean 22 +/- 9); 49 patients were in NYHA class II, and 45 were in NYHA class III. Mean peak VO2 was 17 +/- 5 ml/min/kg. During a follow-up period of 23 +/- 11 months, 21 patients died. The 1-, 2- and 3-year cumulative survival rates for the 94 patients were 88, 79 and 69%, respectively. Functional status, as assessed both by peak VO2 and NYHA classification, and left ventricular ejection fraction were significantly worse in the group of nonsurvivors. The most powerful independent predictor of mortality was peak VO2. Although mean peak VO2 was significantly higher in NYHA class II than in NYHA class III (20 +/- 4 vs 13 +/- 3 ml/min/kg, p less than 0.0001), categorization into subgroups on the basis of the attained peak VO2 revealed a marked discrepancy with the NYHA classification. Nevertheless, the survival curves of patients subdivided at a peak VO2 of 16 ml/min/kg showed a strong resemblance with survival curves of both NYHA classes.(ABSTRACT TRUNCATED AT 250 WORDS)