Background Abnormal findings, including silent cerebral infarction, are frequently observed by magnetic resonance imaging (MRI) in patients with nonvalvular atrial fibrillation (NVAF); however, the prevalence and prevention strategy for these lesions have not been extensively studied. In the present study the preventive effects of aspirin on silent ischemic lesions was investigated. Methods and Results Silent lesions were counted using cranial MRI performed in 78 neurologically normal adults with sinus rhythm and in 212 patients with NVAF without a history of stroke. MRIs were repeated twice in the NVAF patients at 12-month intervals. During the first year, patients received neither antiplatelet agent nor anticoagulant; in the second year, aspirin (330 mg daily) was administered. The prevalence of lesions in the initial MRI was higher in NVAF patients (86.4%) than in sinus rhythm subjects (53.8%; p<0.001). After 12 months without aspirin, new lesions were seen in 20.6% of NVAF patients. The yearly occurrence of new lesions was decreased to 9.6% during the year of treatment with aspirin (p=0.014). Conclusions In patients with NVAF, abnormal lesions are frequently observed by MRI and aspirin treatment may be effective in preventing further small silent lesions. (Circ J 2004; 68: 410 - 416)