We looked for the prevalence of aspirin nonresponders, compared the results of 2 tests assessing aspirin responses - measurement of urinary 11-dehydrothromboxane B2 (dTXB2) and VerifyNow Aspirin assay - in patients with ischemic stroke, and examined the relationship of aspirin nonresponse and the outcomes of the patients. Methods: One hundred one patients with ischemic stroke were prospectively included. Aspirin response was assessed by urinary dTXB2 measurement and VerifyNow Aspirin assay. The Spearman correlation coefficients and kappa statistics were calculated to assess correlation and agreement between the 2 tests. The measured outcome was the occurrence of cardiovascular events and death. Results: Prevalence of aspirin nonresponders was 40% and 6%, if they were measured by urinary dTXB2 and VerifyNow Aspirin assay, respectively. Poor correlation in the results between the 2 tests was found (r =.135, P =.190). The degree of agreement between the 2 tests in relation to resistance status was weak (kappa =.032, P =.590). With a mean follow-up time of 17 months, the outcomes occurred significantly higher in aspirin nonresponders who were diagnosed by urinary dTXB2 measurement as compared with patients with aspirin response (18% versus 2%, odds ratio 8.8, 95% confidence interval 1.18-65.4, P =.037). Conclusions: Our research confirmed poor correlation and lack of agreement between the 2 tests. Only aspirin nonresponders who were diagnosed by dTXB2 measurement were related to having cardiovascular events and death. Further research is still needed to identify the best method of diagnosis of aspirin nonresponders. © 2014 by National Stroke Association. --------------------------------------------------------------------------------