A 47-year-old woman suddenly developed abdominal pain and visited the emergency room of our hospital. Chest and abdominal CT scan revealed free air in the abdominal cavity and a bulky pulmonary tumor in the right middle lobe. The perforated sigmoid colon was removed in an emergency operation. Histological examination of the resected tissue revealed undifferentiated carcinoma, but the histological differentiation could not be determined. We used immunohistochemical staining to distinguish primary non-small cell lung cancer from colon cancer; the resected tumor was positive for TTF-1 and CK7, but negative for CK20. Therefore, by using immunohistochemical staining we could diagnose the tumor of the large intestine as metastasis from non-small cell lung cancer. After the operation, systemic chemotherapy with carboplatin and docetaxel was repeated, but the lung tumor did not regress and the patient died. Autopsy examination confirmed the histology of the lung tumor as pleomorphic carcinoma. Morphological characteristics and the immunohistochemical staining pattern of the pulmonary tumor were consistent with that of the colon tumor. In Japan, this is the first report in which the initial manifestation of lung cancer was perforation of the large intestine due to metastasis.