A 59-year-old man was admitted to our hospital because of sudden weakness in his left foot. He had been treated for lung cancer by chemotherapy and irradiation 3 years earlier. Brain magnetic resonance (MR) imaging revealed multiple acute cerebral infarctions in the area of the right anterior cerebral artery. MR angiography (MRA) revealed that the right anterior cerebral artery was patent, with slight irregularity in the A3 portion. He was treated by administration of aspirin (200 mg/day) and a continuous intravenous unfragmented heparin infusion (10,000 IU/day). Four days after admission, he developed dyspnea. Chest computed tomography (CT) performed 5 days after admission revealed both a marked pericardial effusion and a pleural effusion. Emergency pericardiocentesis was therefore performed. While 1,000 ml of bloody pericardial effusion were aspirated, his dyspnea ameliorated dramatically. Histological examination of the pericardial effusion revealed infiltration of lung adenocarcinoma cells in the pericardium. Intracranial 3D-CT angiography revealed the pearl and string sign in the right anterior cerebral artery 6 days after admission. Anterior cerebral artery dissection was diagnosed as the cause of his cerebral infarction. It is important to recognize the possibility of cardiac tamponade as an uncommon complication of the treatment for acute cerebral infarction.