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Background and Purpose Many studies use medical record review for ascertaining outcomes. One large, longitudinal study, the Women’s Health Initiative (WHI) ascertains strokes using participant self-report and subsequent physician review of medical records. This is resource-intensive. Herein, we assess whether Medicare data can reliably assess stroke events in the WHI. Methods Subjects were WHI participants with fee-for-service Medicare. Four stroke definitions were created for Medicare data using discharge diagnoses in hospitalization claims. Definition 1: stroke codes in any position; Definition 2: primary position stroke codes; Definitions 3 & 4: hemorrhagic and ischemic stroke codes respectively. WHI data were randomly split into training (50%) and test sets. A concordance matrix was used to examine agreement between WHI and Medicare stroke diagnosis. A WHI stroke and a Medicare stroke were considered a match if they occurred within +/− 7 days of each other. Refined analyses excluded Medicare events where medical records were unavailable for comparison. Results Training data (n=24,428): There were 577 WHI strokes and 557 Medicare strokes using definition 1. Of these, 478 were a match. Algorithm performance: Specificity 99.7%; Negative Predictive Value 99.7%; Sensitivity 82.8%; Positive Predictive Value 85.8%; kappa 0.84. Performance was similar for test data. While specificity and negative predictive value exceeded 99%, sensitivity ranged from 75 to 88% and positive predictive value ranged from 80 to 90% across stroke definitions. Conclusion Medicare data appear useful for population-based stroke research; however the performance characteristics depend on the definition selected.