To study whether the relative incidence of sudden death versus progressive congestive heart failure is related to the severity of congestive heart failure as assessed by determination of peak oxygen consumption, we followed 90 ambulatory patients with moderate to severe congestive heart failure for 24.1 +/- 13.1 months. All patients had a left ventricular ejection fraction of 40% or less (mean, 22.6% +/- 9.2%) and a peak oxygen consumption of 20 ml/min/kg or less (mean, 14.7 +/- 3.5 ml/min/kg). Patients with severe congestive heart failure who might be eligible for heart transplantation (group I: n = 37; peak oxygen consumption < or = 14 ml/min/kg, mean, 11.0 +/- 1.8 ml/min/kg; mean left ventricular ejection fraction, 22.3% +/- 9.3%) were compared with those considered too well for heart transplantation (group II: n = 53; peak oxygen consumption > 14 < or = 20 ml/min/kg, mean, 17.1 +/- 1.6 ml/min/kg; mean left ventricular ejection fraction, 22.9% +/- 9.1%). During follow-up, 15 patients (41%) in group I died; 11 patients (21%) in group II died. In group I, seven of the 15 deaths (47%) were sudden; in group II, nine of the 11 deaths (82%) occurred suddenly. Patients who died suddenly had a significantly higher peak oxygen consumption (14.0 +/- 3.5 ml/min/kg) than those who died of progressive congestive heart failure (11.0 +/- 3.1 ml/min/kg; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)