Early detection of atrial fibrillation patients at high risk for stroke is important. There are some studies which indicate that mean platelet volume (MPV) determines the prognosis and risk in patients with a stroke. In this study, our aim was to investigate the association between the MPV measured in stroke patients with atrial fibrillation. Consecutive patients referred to our center between January 2010 and April 2012 were included in this study. The patients with atrial fibrillation were classified into two groups according to presence or absence of a history of stroke by combining data from the medical histories after a thorough review of the medical records. MPV determination was made within 24 h following the onset of stroke. We studied 63 consecutive stroke patients with atrial fibrillation and 77 atrial fibrillation patients without stroke history. In receiver-operating characteristic (ROC) curve analysis, the value for MPV levels to detect stroke with a sensitivity of 63.5% and specificity of 64.4% was 9.4 fI. High MPV (>9.4 fI) was significantly associated with the occurrence of stroke [odds ratio (OR) 4.021, 95% confidence interval (CI) 1709-9464, P<0.001]. Our study supports the hypothesis that a high MPV is associated with an increased risk of stroke in atrial fibrillation patients. Blood Coagul Fibrinolysis 24:55-58 (C) 2012 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.