BACKGROUND: Aspirin is one of the effective antiplatelet agents with proven benefits for the prevention of ischemic stroke. However, ischemic stroke may recur in some patients despite aspirin therapy. We investigated the prevalence of laboratory assessed low-responsiveness to aspirin in patients who are treated with aspirin for secondarily preventing cerebrovascular events, and we did so by using the VerifyNow(R) Aspirin assay. METHODS: We measured the platelet function using the VerifyNow(R) test in the recurrent (RG) and no-recurrent groups (NRG) that were treated with aspirin for secondarily preventing cerebrovascular events, We analyzed the association of a low response to aspirin with the clinical ischemic events and the factors associated with a low response. RESULTS: There were 110 patients on aspirin for secondary prevention and the mean treatment duration was 17 months. The incidence of a low response to aspirin was significantly higher in the RG than that in the NRG (26.2% vs. 5.8%, respectively, p=0.03). Multivariate analysis revealed that smoking was an independent predictor of a low response to aspirin (p=0.003). CONCLUSIONS: We found that up to 26.2% of the patients with recurrent stroke are laboratory assessed aspirin low-responsive (as measured with the VerifyNow(R) Aspirin assay), despite that they are on chronic aspirin therapy. Aspirin lowresponsiveness may be associated with the clinical failure to prevent recurrent ischemic cerebrovascular diseases, and this is known as clinical aspirin low-responsiveness.