The return map is the plot that displays the relationship between a point and its consecutive point in a time series. This study tried to characterize and quantify the return map of RR intervals (RRI) in subjects with patent coronary arteriogram and in patients with acute myocardial infarction (AMI) by using some parameters of linear regression between RRI(n+1) and RRI(n), and compared the newly introduced parameters with the time- and frequency-domain indices of heart rate variability (HRV) obtained from the same RRI. Three quantitative measures were introduced to quantify the return map of RRI: the Pearson coefficient, slope and Y-intercept of linear regression between RRI(n+1) and RRI(n). We found that the Pearson coefficient was almost identical to the slope of linear regression, and correlated significantly and negatively with Y-intercept in controls and patients with AMI. The Pearson coefficient and the slope in patients with AMI were significantly larger while the Y-intercept was significantly smaller than those of controls. The Pearson coefficient was also found to correlate significantly and negatively with the indices of vagal modulation and positively with the indices of sympatho-vagal balance and rennin-angiotensin-aldosterone modulation in both normal controls and patients with AMI. Our results suggested that the return map of RRI could be characterized and quantified by Pearson coefficient, which was related to the autonomic nervous modulation of the subject.