Background It is sometimes difficult to make a diagnosis of cardioembolic stroke in the stroke care unit, because of the splashing and vanishing of the intracardiac source of the emboli on transesophageal echocardiography. Serum fibrin-monomer (FM) is a new marker for coagulation activity that is useful for identifying older individuals at increased risk of ischemic stroke. Methods and Results Two hundred and four patients with acute ischemic stroke were examined for serum coagulation and fibrinolytic activity on admission, and underwent transesophageal echocardiography within 7 days of onset. Serum levels of FM was significantly higher in patients with left atrial appendage (LAA) thrombus formation (n=24) than in those with no thrombus (88±52 vs 14±9 μg/ml, p<0.0001). On multivariate logistic regression analysis, FM was an independent predictor for LAA thrombus (RR 2.975, 95% confidence interval 1.114 to 4.820, p=0.0214). In patients with LAA thrombus negative group, cases with LAA emptying flow velocity at atrial systole that was absent or smaller than at early diastole had significantly higher FM levels as compared to cases with larger velocity (42±12 vs 8±5 μg/ml, p<0.0001). Conclusion Higher levels of serum FM reflect LAA flow pattern alterations and thrombus formation in patients with acute ischemic stroke. (Circ J 2006; 70: 971 - 976)