A 72-year-old man who received warfarin for myocardial infarction (prothrombin time-international normalized ratio [PT-INR] controlled between 2.2 and 2.5) for 2 years. He developed lung cancer, underwent surgery, and received tegafur plus uracil (UFT) after 1 month. After 2 months, he was admitted for hemoptysis and dyspnea. Chest radiography and computed tomography showed bilateral alveolar infiltration (PT-INR, 8.9). Bronchoalveolar lavage fluid (BALF) disclosed hemorrhagic features in sequential samples. And he was diagnosed with diffuse alveolar hemorrhage (DAH). A known interaction exists between fluoropyrimidines and warfarin. So, they were discontinued, and vitamin K was intravenously administered. One day later, the PT-INR returned to 1.14. The symptoms improved and, alveolar infiltration resolved after 2 weeks. Alveolar hemorrhage may be due to an interaction between UFT and warfarin. When fluoropyrimidines and warfarin are prescribed simultaneously, we recommend that PT-INR should be closely monitored.