To examine the predictive value of ventricular arrhythmias on ambulatory electrocardiographic (ECG) monitoring, 211 patients with left ventricular dysfunction and congestive heart failure (76% men, age 63 +/- 4 years, left ventricular ejection fraction 0.26 +/- 0.10) were studied. During a follow-up of 21 +/- 11 months, there were 45 cardiac deaths: 22 were due to progressive pump failure and 23 were sudden. Patients with a low left ventricular ejection fraction (< or = 0.27) and ventricular tachycardia on 24 h ECG were at higher risk of dying suddenly and from progressive pump failure (both P < 0.0001). Patients who died suddenly were found to have significantly longer (P = 0.003) and faster (P = 0.029) ventricular tachycardias on their baseline ambulatory ECG, than survivors. This association was not observed in patients who died of progressive pump failure. Therefore, low left ventricular ejection fraction and ventricular tachycardia on 24 h ECG recording predict an increased risk of cardiac mortality. Our results also suggest that longer and faster ventricular tachycardia recorded by 24 h ECG may identify patients at risk of sudden death, a finding which has not been described before.