The ability to record ambulatory electrocardiograms, as initially developed by Holter,1 was an important step in medical technology. Until now, however, the usefulness of this procedure has been hampered by the difficulty in evaluating the recordings themselves.2 Not only is the qualitative evaluation directly related to the skill of the individual handling the processing, but quantitative evaluation is essentially impossible. As a result, techniques that maximize the information obtained from ambulatory electrocardiograms3 have become increasingly important-particularly in evaluating the role of arrhythmias in sudden death, the independent prognostic value of arrhythmias in the post-infarction period, and the response of arrhythmias to drugs.