In two patients with old myocardial infarction and congestive heart failure there was a progressive reduction of heart rate variability and eventual sudden death. The two patients had had three 24 h electrocardiogram recordings within two years of death. The power of two spectral bands was calculated-0.04-0.15 Hz, low frequency power, and 0.15-0.40 Hz, high frequency power. The mean low and high frequency powers over the 24 h recording progressively decreased whereas the frequency of ventricular arrhythmias showed no consistent changes in either patient. The circadian variation in hourly low and high frequency powers in the last 24 h electrocardiogram recordings was much reduced. These results suggest that sequential measurements of heart rate variability may be useful in predicting sudden death.