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Introduction: Cerebral ischemia and vascular risk factors are associated with the development of Alzheimer disease (AD). While Parkinson disease (PD) is also a common neurodegenerative condition, the relationship between ischemic stroke and PD remains unclear. Some evidence suggests a shared pathogenic pathway between both diseases.Methods: We used inpatient and outpatient claims data from 2008-2014 in a 5% sample of Medicare beneficiaries ≥66 years of age. Our variables of interest were: 1) a hospital-based diagnosis of ischemic stroke and 2) an outpatient or hospital-based diagnosis of idiopathic PD. Previously validated ICD-9-CM code algorithms were used to identify all diagnoses. We used Cox proportional hazards modeling to characterize the relationship between ischemic stroke and PD, while adjusting for demographics and vascular risk factors. We assessed both the association between PD and subsequent stroke, as well as stroke and subsequent PD. In a separate but identically designed set of analyses, we characterized the relationship between ischemic stroke and AD as a point of comparison.Results: Our analysis encompassed nearly 1.6 million patients with a mean age of 73(+/- 8) years, of whom 57% were female. The annual incidence of ischemic stroke was 1.75% (95% confidence interval [CI], 1.67-1.85%) after a diagnosis of PD versus 0.96% (95% CI, 0.96-0.97%) in those without PD (adjusted hazard ratio [aHR], 1.25; 95% CI, 1.19-1.32). In contrast, the annual incidence of ischemic stroke was 1.96% (95% CI, 1.89-2.03%) after a diagnosis of AD versus 0.96% (95% CI, 0.96-0.97%) in those without AD (aHR, 0.98; 95% CI, 0.95-1.02). The annual incidence of PD was 0.97% (95% CI, 0.92-1.03%) after ischemic stroke versus 0.39% (95% CI, 0.38-0.39%) in those without ischemic stroke (aHR, 1.62; 95% CI, 1.53-1.72). In contrast, the annual incidence of AD was 3.66% (95% CI, 3.56-3.78%) after a diagnosis of ischemic stroke versus 1.17% (95% CI, 1.16-1.17%) in those without ischemic stroke (aHR, 1.67; 95% CI, 1.61-1.72).Conclusions: Among Medicare beneficiaries, the relationships between stroke and PD were similar to those between stroke and AD. As in AD, a link may exist between cerebrovascular disease and PD.