An in-hospital diagnosis of heart failure due to left ventricular dysfunction carries a very poor prognosis, which has been reported to be as high as 50% per annum. In many cases death has been sudden and unexpected and not strictly related to a refractory heart failure. In association with this, it has become more widely appreciated that ventricular tachyarrhythmias, often asymptomatic, are frequent in patients with left ventricular dysfunction, and in some studies, the frequency of ventricular arrhythmias has been the most powerful predictor of mortality. Several other variables have been reported as being predictors of a poor prognosis, including electrolyte abnormalities, poor left ventricular function, high plasma norepinephrine, and low blood pressure. In a prospective follow-up series of 152 patients with heart failure secondary to left ventricular dysfunction, we found that ventricular arrhythmias were the strongest predictor of subsequent mortality, while treatment with amiodarone was associated with longer survival. High plasma renin and norepinephrine concentrations were also associated with a poor prognosis and both were directly correlated with the frequency of ventricular arrhythmias. In previous double-blind studies we have demonstrated a reduction in ventricular arrhythmias with both captopril and enalapril, and this has been associated with a reduction in plasma norepinephrine and an increase in serum and total body potassium. Nonetheless, arrhythmias in these patients were by no means abolished despite correction of these neuroendocrine and electrolyte abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)