BACKGROUND: There are many causes for the failure of aspirin therapy in patients with acute ischemic stroke. Laboratory aspirin resistance (AR) might be involved in clinical aspirin non-response. The PFA-100 is a laboratory method to evaluate AR in the clinical setting. However, there has been limited data regarding concordance with optical platelet aggregometry, which is considered the gold standard for detecting AR. We retrospectively analyzed platelet function tests using the PFA-100 and an optical platelet aggregometer in 86 patients with acute ischemic stroke. METHODS: Eighty six patients were enrolled in the study and were evaluated the platelet function test by optical aggregometer and a PFA-100. We determined the variability in the prevalence of AR and the kappa value between the two tests in patients with acute ischemic stroke. RESULTS: Among 86 patients, 27 (31.4%) were detected as AR by the optical aggregometer and 31 (36.0%) by the PFA-100. There were 13 cases of AR (15.1%) in both laboratory methods. The optical platelet aggregometer results showed that female gender (P=0.03), aspirin monotherapy (P =0.05), and NIHSS at baseline (P=0.04) were related with AR in acute ischemic stroke. Multiple logistic regression analysis showed that NIHSS was independently associated with AR of the optical platelet aggregometer (OR=1.12 95%, CI: 1.00-1.25, P=0.05). CONCLUSION: The prevalence of AR was similar between the PFA-100 and the optical platelet aggregometer in patients with acute ischemic stroke. However, the concordance rate of these two tools is low.