Recent interest in autonomic nervous system dysfunction and its effect on mortality from cardiac events has led to power spectrum ECG measurements. There seems to be a significant relationship between decreased heart rate variability and mortality in ECGs recorded from patients following myocardial infarction. Further autonomic nervous system activity in patients can be evaluated by spectral frequency analysis of the ECG. This analysis is divided into ultra low, very low, low, and high frequencies. Such analysis demonstrates a marked decrease in variability in all four frequency categories in postmyocardial infarct patients. It is postulated that high-frequency power and heart rate variability are modulated by the parasympathetic nervous system, whereas low-frequency power is modulated by both the sympathetic and parasympathetic systems. This effect appears to last for at least 12 months following infarction. The recovery of normal heart rate variability and the declining rate of mortality following infarction appear to occur nearly simultaneously, although further analysis is needed to rule out a coincidental finding.