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Objective: Psychological traits such as optimism and hostility affect coronary heart disease (CHD) risk, but mechanisms for this association are unclear. We hypothesized that optimism and hostility may affect CHD risk via changes in heart rate variability (HRV)., Methods: We conducted a longitudinal analysis using data from the Women's Health Initiative Myocardial Ischemia and Migraine Study. Participants underwent 24-hour ambulatory electrocardiogram monitoring 3 years after enrollment. Optimism (Life Orientation Test-Revised), cynical hostility (Cook-Medley), demographics, and coronary risk factors were assessed at baseline. HRV measures included standard deviation of average N-N intervals (SDNN); standard deviation of average N-N intervals for 5 minutes (SDANN); and average heart rate (HR). CHD was defined as the first occurrence of myocardial infarction, angina, coronary angioplasty, and bypass grafting. Linear and Cox regression models adjusted for CHD risk factors were used to examine, respectively, associations between optimism, hostility, and HRV and between HRV and CHD risk., Results: Final analyses included 2655 women. Although optimism was not associated with HRV, hostility was inversely associated with HRV 3 years later (SDANN: adjusted β = -0.54; 95% CI = -0.97 to -0.11; SDNN: -0.49; 95% CI = -0.93 to -0.05). HRV was inversely associated with CHD risk; for each 10-millisecond increase in SDNN or SDANN, there was a decrease in CHD risk of 9% (p = .023) and 12% (p = .006), respectively., Conclusions: HRV did not play a major role in explaining why more optimistic women seem to be somewhat protected from CHD risk. Although hostility was inversely associated with HRV, its role in explaining the association between hostility and CHD risk remains to be established.