The patient was a 75-year-old female with a history of no smoking. Under a diagnosis of lung cancer, she underwent a right lower lobectomy in March 2008. She was started on oral Uracil-Tegafur (UFT) (400 mg/day) from April and in May developed fatigue, respiratory discomfort, and tachycardiac atrial fibrillation. Chest X-ray film showed an increase in right pleural effusion. Thoracentesis revealed a yellowish, serous exudate containing predominantly lymphocytes, with no evidence of malignancy. Despite continued diuretic administration for 5 months from July, it was difficult to control the pleural effusion, and her activities of daily living remained low. In December of the same year, the oral administration of UFT was terminated, which 2 weeks later resulted in a marked decrease in pleural effusion on chest X-ray film. Respiratory discomfort and fatigue also subsided, and her general condition improved markedly. Herein we report a case of oral UFT-induced pleural effusion following lung cancer surgery.