The study of heart rate variability is a new means of assessing autonomic nervous system function and the risks of cardiac and sudden death in patients with advandec cardiac disease. The aim of this study was to analyse changes in heart rate variability in the hours preceding cardiac death and just before its occurrence. Seventeen subjects aged 78.5 +/- 10 years, with advanced cardiac disease responsible for a reduction of left ventricular ejection fraction below 40%, died during Holter ECG recording. Ten died of ventricular fibrillation, 5 of bradycardia and 2 of non-rhythmic causes. General analysis of heart rate variability showed a decrease in all but 1 patient, the average standard deviation of normal RR intervals in subjects in sinus rhythm being 53 +/- 14 msec and the fractioned spectral power (low frequency/high frequency power) being 1 +/- .07. The change in heart rate variability did not allow prediction of the mechanism of death: the mean heart rate only increased before death in 3 of the patients with an ischaemic component in the hours before death, the indices of vagal tone were very low in the majority of patients and, just before death, a disequilibrium between the spectral powers with a sudden increase in LF/HF ratio of 10 of the 17 patients, irrespective of the cause of death, appeared. In conclusion, a decrease in heart rate variability was observed in all cases but did not predict the mechanism of death. Just before death occurred, some patients, especially those with acute ischaemia, showed a sudden change in the indices of heart rate variability indicating a terminal vaso-sympathetic disequilibrium.