We report the rare and interesting case of a primary lung cancer detected 5 years after cancer of unknown primary (CUP) of a mediastinal lymph node (LN) was resected. A 40-year-old male was diagnosed with adenocarcinoma of unknown primary in a mediastinal lymph node after resection of the mediastinal tumor. Five years after resection of the CUP in mediastinal LN, a small, abnormal nodular shadow in left upper lobe was detected by chest CT. This pulmonary tumor was diagnosed as a lung adenocarcinoma. The pathological and immunohistological findings of the resected pulmonary tumor resembled those of the LN resected 5 years before. We speculated that the pulmonary lesion represented primary lung cancer that enlarged later than the metastatic mediastinal LN. This case illustrates the importance of careful observation and long-term follow-up in patients treated for CUP of a thoracic LN.