It can be difficult to distinguish between primary and metastatic squamous cell carcinoma (SCC) in the lung. Surgical specimens were obtained from two groups of patients, 26 lung SCC patients without histories of any other cancer (the definite primary group) and 17 patients who had undergone surgical removal of SCC emerging in the lung after surgery for tongue SCC (the unknown group). From the former, 26 primary lung SCC were obtained. From the latter, 17 lung tumors and 15 primary tumors of the tongue were obtained. Eleven of the 17 lung tumors from the unknown group were metastatic lung SCC. All specimens were immunostained with cytokeratin (CK)5/6, CK7, CAM5.2, CK19 and p63 antibodies. The frequency of CAM5.2 and CK19 expression was significantly higher in the lung SCC of the definite primary group (21 of 26, 81% and 20 of 26, 78%, respectively) than in the metastatic lung SCC (1 of 11, 9% (P < 0.001) and 2 of 11, 18% (P = 0.003), respectively) or primary SCC of the tongue (5 of 15, 33% (P = 0.002) and 2 of 15, 13% (P < 0.001), respectively). CAM5.2 and CK19 are useful for distinguishing between primary SCC of the lung and metastases from tongue cancer.