Recent studies have suggested that hyperviscosity is frequent in patients with atrial fibrillation (AF). The aims of this study were to evaluate if hemorheologic alterations play a role in the occurrence of cerebral ischemic events in patients with AF and to explore a possible association between inflammation and hyperviscosity in these patients. Sixty-two patients with AF with a history of >or=1 cerebral ischemic event and 94 patients with AF without cerebral ischemic events were studied. A control population included 130 age- and gender-matched healthy volunteers. Hemorheologic variables (whole-blood viscosity, plasma viscosity, the erythrocyte deformability index, and hematocrit), fibrinogen, and high-sensitivity C-reactive protein levels were assayed. An alteration in whole blood viscosity at 94.5 seconds(-1) and the erythrocyte deformability index were found more frequently in patients with previous ischemic events on univariate and multivariate analyses (odds ratio 3.19, p=0.023 and odds ratio 4.26, p=0.002, respectively) adjusted for age, gender, hypertension, diabetes, history of coronary artery disease, left ventricular dysfunction, smoking habit, dyslipidemia, hematocrit, fibrinogen, high-sensitivity C-reactive protein, and hemorheologic parameters. These results should stimulate prospective studies on the role of hemorheologic alterations in the occurrence of cerebral ischemic complications in patients with AF.