Case 1 was a 56-year-old man with Stage IIIB squamous cell carcinoma of the lung. He had an abdominal pain, and a CT scan suggested perforation of the digestive tract. His general condition was improved with conservative medical management. He died 50 days after the perforation as a result of the lung carcinoma's progress. An autopsy revealed perforation by small intestine metastasis from lung cancer. Case 2 was a 64-year-old man with Stage IV pleomorphic carcinoma of the lung. Because of the progressive disease after two courses of chemotherapy, he was under the best supportive care. A CT scan of the abdomen showed a small intestine metastasis. He had a sudden onset of epigastralgia. Free air detected by the CT scan suggested perforation by the small intestine metastasis. With conservative medical management, he died 70 days after the perforation. Case 3 was an 87-year-old man with Stage IB large-cell carcinoma of the lung. He had an upper abdominal pain and the free air turned out in a CT scan. He underwent an emergency laparotomy, and the surgical findings showed perforation by the small intestine metastasis. He died 18 days after the operation. When the patients have abdominal symptoms, it is important to consider the possibility of perforation by the small intestine metastasis.