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Summary. Background: Protein C is an important plasma natural anticoagulant. Although protein C deficiency increases risk of venous thrombosis, it remains uncertain whether low protein C increases risk of atherothrombosis. Objective: To examine whether low protein C may be a risk factor for ischemic stroke or coronary events in a prospective population-based study. Patients/Methods: The Atherosclerosis Risk in Communities Study assessed protein C antigen by ELISA at baseline in 1987–89 and followed participants (n = 13 879) for incident ischemic stroke or coronary events through 2005. Results: Over a median of 16.9 years of follow-up, 613 ischemic strokes and 1257 coronary heart disease events occurred. Protein C was inversely associated with incidence of ischemic stroke. Adjusted for multiple risk factors, the rate ratios (95% CIs) from highest to lowest quintiles were 1.0, 1.16 (0.90–1.50), 1.22 (0.94–1.58), 1.18 (0.90–1.55) and 1.52 (1.17–1.98). This inverse association was stronger for non-lacunar and cardioembolic stroke than for lacunar stroke. In contrast, there was a positive association between protein C and coronary heart disease in incompletely adjusted models, but no association after adjustment for plasma lipids. Conclusions: In this cohort study, low protein C was a risk factor for incident ischemic stroke but not coronary heart disease. Levels of protein C associated with stroke risk were not restricted to the traditional ‘deficient’ range for protein C (< 0.5 percentile), suggesting that other etiologies for a lower protein C, or genetic variants associated with more subtle changes in protein C, are playing a role in disease pathogenesis.