Introduction: The risk of thromboembolic events in atrial fibrillation (AF) is partially independent of the arrhythmia itself. Endothelial dysfunction could be a pathogenic factor of this phenomenon.Method: The study included 60 patients with recent onset AF (up to 48h), candidates for pharmacological cardioversion. Patients underwent bFMD examination and markers associated with endothelial function (vWF, sE-selectin, ADMA, SDMA, OPG, PTX3, TRAIL, hsCRP) were determined. Patients were re-examined after 7-10 days, using the same assay.Results: The biochemical markers indicated vascular endothelial dysfunction and were partially associated with the presence of co-morbidities. The persistance of atrial fibrillation was associated with a significant decrease in the bFMD while cardioversion to sinus rhythm was not connected with improvement of this parameter. Test markers were not predictors of sinus rhythm return, but allow to predict the presence of the indications for anticoagulation.Conclusions: Patients with acute onset AF are characterized by endothelial dysfunction. The longer duration of arrhythmia leading to further deterioration of endothelial function while the successful cardioversion does not bring the improvement in the short term. The parameters of endothelial function may help to identification of patients with indications for chronic anticoagulation.