Reduced heart rate variability (HRV) is an independent risk factor after myocardial infarction, indicating higher risk of fatal and nonfatal arrhythmias and of cardiac death in general. Analysis of HRV is also a valuable tool in clinical research, providing a non-invasive measurement of fluctuations in sympathetic and parasympathetic activation. We expect increased use of these methods, since new 24-hour: ECG-monitoring equipment is to be delivered with software for HRV as an option. Statistical time domain analysis of the whole 24-hour system of recording is very suitable for stratifying risk after myocardial infarction. Frequency domain analysis requires manual as well as automatic editing, but is the method of choice in clinical research involving short-term recordings and standardized conditions.