Metabolic neuropathophysiology underlying the prediabetic state may confer susceptibility to the adverse health effects of ambient particulate matter <10 microm in diameter (PM(10)). The authors therefore examined whether impaired glucose homeostasis modifies the effect of PM(10) on heart rate variability in a stratified, random sample of 4,295 Women's Health Initiative clinical trial participants, among whom electrocardiograms and fasting blood draws were repeated at 3-year intervals from 1993 to 2004. In multilevel, mixed models weighted for sampling design and adjusted for clinical and environmental covariables, PM(10) exposure was inversely associated with heart rate variability. Inverse PM(10)-heart rate variability associations were strongest for the root mean square of successive differences in normal-to-normal RR intervals (RMSSD). Among participants with impaired fasting glucose, there were -8.3% (95% confidence interval: -13.9, -2.4) versus -0.6% (95% confidence interval: -2.4, 1.3), -8.4% (95% confidence interval: -13.8, -2.7) versus -0.3% (95% confidence interval: -2.1, 1.6), and -4.3% (95% confidence interval: -9.4, 1.0) versus -0.8% (95% confidence interval: -2.7, 1.0) decreases in the RMSSD per 10-microg/m(3) increase in PM(10) at high versus low levels of insulin (P < 0.01), insulin resistance (P < 0.01), and glucose (P = 0.16), respectively. These associations were stronger among participants with diabetes and weaker among those without diabetes or impaired fasting glucose. The findings suggest that insulin and insulin resistance exacerbate the adverse effect of PM(10) on cardiac autonomic control and thus risk of coronary heart disease among nondiabetic, postmenopausal women with impaired fasting glucose.