The activity of the cardiac autonomic nervous system can be estimated by measurement of beat to beat variations in heart rate-heart rate variability (HRV). In survivors after myocardial infarction, reduced 24-hour HRV, is an independent predictor of mortality/sudden cardiac death. The attenuated HRV is presumed to indicate reduced vagal function and concomitant high sympathetic activity. In experimental studies, this combination reduces the threshold for inducing malignant tachyarrhythmias, and is very probably a clinically important factor for the evolution of ventricular arrhythmias. Measurement of HRV in ordinary 24-hour ECG recordings can, together with other non-invasive measurements, stratify arrhythmic risk in survivors of myocardial infarction.