An abnormal shadow was observed on chest X-ray films of a 63-year-old man presenting with cough and sputum. Chest computed tomographic scans disclosed enlargement of the right hilar lymph nodes, but no obvious primary lesion was found in the lung field. Bronchoscopic examination revealed a slightly widened second carina, but no malignant cells were detected by transbronchial aspiration cytology. At surgery, a tumor was found between the truncus superior and the truncus intermedius. The pathologic diagnosis was a metastatic lymph node of poorly differentiated squamous cell carcinoma. Because the tumor severely adhered to the bronchus and pulmonary arteries, we performed a right pneumonectomy with mediastinal node dissection. Pre- and postoperative examinations did not detect the primary lesion, and no recurrence had been observed 76 months after surgery. This was thought to be a very rare case of T0N1M0 lung cancer. In general, the prognosis is poor for patients with metastatic carcinoma of unknown primary site. However, patients with T0 lung cancer, as in this case, might enjoy a better prognosis if complete resection and dissection of metastatic lymph nodes are performed.