Background Previous studies have found associations between fine particulate matter <2.5 µm in diameter (PM2.5) and increased risk of cardiovascular disease (CVD) among populations with no CVD history. Less is understood about susceptibility of adults with a history of CVD and subsequent PM2.5-related CVD events and whether current regulation levels for PM2.5 are protective for this population. Methods and Results This retrospective cohort study included 96 582 Kaiser Permanente Northern California adults with a history of stroke or acute myocardial infarction. Outcome, covariate, and address data obtained from electronic health records were linked to time-varying 1-year mean PM2.5 exposure estimates based on residential locations. Cox proportional hazard models estimated risks of stroke, acute myocardial infarction, and cardiovascular mortality associated with PM2.5 exposure, adjusting for multiple covariates. Secondary analyses estimated risks below federal and state regulation levels (12 µg/m3 for 1-year mean PM2.5). A 10-µg/m3 increase in 1-year mean PM2.5 exposure was associated with an increase in risk of cardiovascular mortality (hazard ratio [HR], 1.20; 95% CI, 1.11-1.30), but no increase in risk of stroke or acute myocardial infarction. Analyses of <12 µg/m3 showed increased risk for CVD mortality (HR, 2.31; 95% CI, 1.96-2.71), stroke (HR, 1.41; 95% CI, 1.09-1.83]), and acute myocardial infarction (HR, 1.51; 95% CI, 1.21-1.89) per 10-µg/m3 increase in 1-year mean PM2.5. Conclusions Adults with a history of CVD are susceptible to the effects of PM2.5 exposure, particularly on CVD mortality. Increased risks observed at exposure levels <12 µg/m3 highlight that current PM2.5 regulation levels may not be protective for this susceptible population.