Cardiac chaos is an emerging paradigm in cardiology that may prove useful in the management of patients with congestive heart failure (CHF). In short, cardiac chaos refers to the minute-yet seemingly purposeful-fluctuations in the instantaneous heart rate or interbeat interval. Recent studies have demonstrated that cardiac chaos accounts for much of the beat-to-beat variability of the heart rate in healthy subjects, but the cardiac chaos appears to subside in patients with CHF. This brief review gives an overview of the background and current understanding about cardiac chaos and its relation to heart rate variability (HRV). Practical considerations on the measurement of cardiac chaos and its clinical application are discussed. (c) 1999 by CHF, Inc.