A 63-year-old man who had undergone resection of colon cancer 15 years previously was found to have a right hilar mass on chest X-ray, and an elevated serum carcinoembryonic antigen level. The hilar lymph nodes were resected with the right upper lobe, and the initial diagnosis was colon cancer metastasis to the right hilar lymph nodes. Although the resection was incomplete, and no additional treatment was given, the patient remained free of recurrence for 10 years. This prompted us to reconsider our diagnosis using immunohistochemistry. The resected lymph nodes were found to be positive for thyroid transcription factor 1 (TTF-1) and cytokeratin 7, and negative for surfactant apoprotein (SAP), cytokeratin 20, and napsin A. The neuroendocrine markers and thyroglobulin were also negative. These findings led us to diagnose T0N1 lung cancer. There are reports of patients with clinical T0N1,2 lung cancer having exceptionally good prognoses despite noncurative treatment; however, to our knowledge, this is the first case of a patient with T0N1 lung cancer diagnosed by immunohistochemistry, with a good prognosis despite incomplete resection. In this case, TTF-1 and cytokeratin staining was particularly helpful in the differential diagnosis.