The association of depression status and severity to 1-year major adverse cardiac events (MACE) and allcause mortality (ACM) after acute coronary syndrome (ACS) is strong and independent of other known risk factors. However, our understanding of the mechanisms underlying this association remains limited. This proposed ancillary study will build on our parent program project, Depression, Biobehavioral Mechanisms, & CHD/Mortality Outcomes. The parent study tests medication adherence, platelet aggregation, and inflammatory markers as possible mechanisms explaining depression's excess MACE/ACM risk. However, recent, exciting findings suggest that depressed, stable cardiac patients' excess MACE/ACM risk is largely explained by less physical activity. This mechanism is not being tested in the parent study. We propose adding an objective, continuous, 90-day assessment of physical activity, using actigraphy, to a new cohort study of 1400 patients enrolled in hospital at the time of an ACS event. This, in combination with the structured interview diagnosis of clinical depression, the assessments of depressive symptoms and selfreported physical activity, and the 12-month tracking of all subjects for MACE/ACM that are already included in the parent study, will enable us to test the hypothesis that physical (in)activity mediates the relationship between depression and cardiac outcome risk. The addition of actigraphy to the parent study is time-sensitive, as we cannot collect these data retrospectively. The parent study benefits by adding one more mechanism to be explored. If confirmed, this will extend the generalizability of the recent findings of stable cardiac patients to a post-ACS population. Specifically, we will test that:1) Patients who are depressed peri-ACS engage in less physical activity during the subsequent 3 months; 2) Lower levels of physical activity during the 3 months post-discharge are associated with increased risk for MACE/ACM during the subsequent 9 months; and 3) The independent association between depression and MACE/ACM is substantially reduced when physical activity is statistically controlled, supporting the role of physical activity as a mediator. Innovation: Having ACS patients steadily increase their physical activity after hospital discharge is a major component of treatment guidelines; to our knowledge, this will be the first study to use actigraphy to continuously monitor physical activity for an extended period following hospital discharge. PUBLIC HEALTH RELEVANCE: Physical activity post-ACS is modifiable, and its lack is a huge public health problem in its own right. If depressed patients are less physically active by objective assessment following ACS hospitalization, and this explains a substantial portion of their excess risk, we will have foundational observational data to propose an exercise intervention in this patient population.