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Introduction: Although the inverse relationship between physical activity (PA) and cardiovascular disease (CVD) is well established, observational studies suggest that very high levels of PA may be harmful. This study sought to understand the relationship between PA, coronary artery calcium (CAC), and cardiovascular outcomes among individuals at different levels of risk., Methods: PA and CAC were assessed in 6777 baseline participants of the Multi-Ethnic Study of Atherosclerosis. Total PA in MET-minutes per week was categorized into quartiles, and CAC was categorized as "low risk" (<100 Agatston units; n = 5180) and "high risk" (≥100 Agatston units; n = 1597). Cox proportional hazard regression analyses and Kaplan-Meier curves were generated to understand relationships between PA and CAC with CVD and all-cause mortality., Results: In low-risk participants in the highest PA quartile, there was a decrease in the adjusted hazard ratio (HR) for CVD (HR, 0.72; 95% confidence interval (CI), 0.56-0.94) and all-cause mortality (HR, 0.69; 95% CI, 0.57-0.84) compared with those in the lowest PA quartile. In high-risk participants in the highest PA quartile, there was a decrease in the adjusted HR for all-cause mortality (HR, 0.59; 95% CI, 0.47-0.74) compared with those in the lowest PA quartile. High PA was not associated with an increased risk of either outcome, regardless of CAC category, sex, or race/ethnicity., Conclusions: Our research suggests that there is no increased risk associated with high levels of PA, even among individuals at high risk of CVD., (Copyright © 2021 by the American College of Sports Medicine.)