In patients with congestive heart failure the frequency of ventricular arrhythmias poorly predicts mortality. It is unknown whether the length of ventricular tachycardia is a better predictor of mortality in these patients. We therefore investigated the prognostic importance of the length of the longest run of ventricular tachycardia, the frequency of ventricular tachycardia, and the frequency of ventricular premature depolarizations with 24-hour ambulatory electrocardiographic recordings in 122 patients with heart failure. We also determined whether the cause of heart failure affects the prognostic importance of these parameters. Each ambulatory electrocardiographic recording was evaluated for the frequency of ventricular premature depolarizations and ventricular tachycardia and for the length of the longest run of ventricular tachycardia. For each electrocardiographic parameter patients were divided into groups based on the median value of that parameter. Mortality among groups was compared in all patients and then separately for nonischemic and ischemic patients. Neither the frequency of ventricular premature depolarizations nor the frequency of ventricular tachycardia predicted mortality whether or not cause was considered. When all patients were examined, the length of ventricular tachycardia did predict an increased risk of death. However, when cause was considered, length of ventricular tachycardia predicted mortality only in the nonischemic patients and not in the ischemic patients. We conclude that the length of ventricular tachycardia may be the best electrocardiographic predictor of mortality in patients with nonischemic heart failure.