It is still unknown whether the associations between particulate matter (PM) and heart rate variability (HRV) differ by particle sizes with aerodynamic diameters between 0.3 microm and 1.0 microm (PM(0.3-1.0)), between 1.0 microm and 2.5 microm (PM(1.0-2.5)), and between 2.5 microm and 10 microm (PM(2.5-10)). We measured electrocardiographics and PM exposures in 10 patients with coronary heart disease and 16 patients with either prehypertension or hypertension. The outcome variables were standard deviation of all normal-to-normal (NN) intervals (SDNN), the square root of the mean of the sum of the squares of differences between adjacent NN intervals (r-MSSD), low frequency (LF; 0.04-0.15 Hz), high frequency (HF; 0.15-0.40 Hz), and LF:HF ratio for HRV. The pollution variables were mass concentrations of PM(0.3-1.0), PM(1.0-2.5), and PM(2.5-10). We used linear mixed-effects models to examine the association between PM exposures and log10-transformed HRV indices, adjusting for key personal and environmental attributes. We found that PM(0.3-1.0) exposures at 1- to 4-hr moving averages were associated with SDNN and r-MSSD in both cardiac and hypertensive patients. For an interquartile increase in PM(0.3-1.0), there were 1.49-4.88% decreases in SDNN and 2.73-8.25% decreases in r-MSSD. PM(0.3-1.0) exposures were also associated with decreases in LF and HF for hypertensive patients at 1- to 3-hr moving averages except for cardiac patients at moving averages of 2 or 3 hr. By contrast, we found that HRV was not associated with either PM(1.0-2.5) or PM(2.5-10). HRV reduction in susceptible population was associated with PM(0.3-1.0) but was not associated with either PM(1.0-2.5) or PM(2.5-10).