A 69-year-old male presented with a persistent cough. Chest computed tomography (CT) demonstrated a large tumor 9.5 × 8.2 cm, in the mediastinum. Because fiberoptic bronchoscopy was insufficient for diagnosis, mediastinoscopic tumor biopsy under general anesthesia was undertaken. Histological examination revealed adenocarcinoma. Despite the extensive diagnostic work-up, including gastroscopic and colonoscopic examinations, enhanced CT of the body, and PET/CT, no primary cancer site was detected. Mediastinal lymph node carcinoma of unknown primary site was diagnosed. The patient was treated with docetaxel and cisplatin with concurrent thoracic radiation therapy. A month after the start of chemoradiotherapy, the mediastinal tumor regressed markedly. Three months after the start of chemoradiation therapy, the patient had radiation pneumonitis which was treated methylprednisolone successfully. About 4 years after the start of therapy, the patient remained free of symptoms without regrowth of the primary site. There have been only a few reports showing the effectiveness of chemoradiotherapy in the treatment of mediastinal lymph node carcinoma from an unknown origin. It is suggested that appropriate chemoradiotherapy for mediastinal lymph node carcinoma of an unknown primary site may offer a chance of long-term survival in patients who are not eligible for radical exsection.