Aspirin resistance occurs in 5-45% of high-risk patients, with various mechanisms proposed for its development. This study aimed to determine the relationships among aspirin resistance, aspirin dosage, type of aspirin and glycoprotein IIIa P1A1/A2 polymorphism in patients with vascular risk factors. Two hundred and eight (75 symptomatic, 133 asymptomatic) patients with vascular risk factors who were using aspirin for primary or secondary prevention were prospectively included. The symptomatic group was further classified into two groups according to aspirin use at the time of stroke. Aspirin resistance was measured by the PFA-100 system (collagen/epinephrine cartridge) and glycoprotein IIIa P1A1/A2 polymorphism was determined by PCR. The overall prevalence of aspirin resistance was 32.2%. The mean age of patients with aspirin resistance was significantly higher than that in those who did not have resistance (P=0.009). The prevalence of aspirin resistance was similar for the symptomatic and asymptomatic under aspirin therapy groups. The resistance rate was found to be highest with 100mg enteric-coated preparation use (39.3%). Increasing the aspirin dosage and/or shifting to uncoated preparations caused a change in aspirin sensitivity of 36-60%. Repeated measurements showed development of aspirin resistance in 14% of patients who were sensitive to aspirin in previous measurements. Glycoprotein IIIaP1A1/A2 polymorphism, aspirin resistance and development of atherothrombotic stroke were not significantly related. The effect of aspirin can change by time, dosage and type of preparation used. There are no relationships among glycoprotein IIIa P1A1/A2 polymorphism, aspirin resistance and development of atherothrombotic stroke. Copyright (C) 2016 Wolters Kluwer Health, Inc. All rights reserved.