A 57-year-old woman presented with the complaint of chronic cough. She had no history of hemoptysis, night sweats, anorexia, dysphagia, vomiting or epigastric pain. Computed tomography showed multiple mediastinal lymph nodes (LNs). The bronchoscopic examination showed no endobronchial lesions. Endoscopic ultrasonography easily visualized the mediastinal LNs, and endoscopic ultrasonography-guided fine needle aspiration of subcarinal lymph node provided tissue diagnosis of a mucinous adenocarcinoma consistent with gastrointestinal origin. Although the patient denied any stomach complaints, subsequent gastroscopy revealed a primary gastric malignancy. The current case is unique in that a pulmonary symptom was the presenting sign of an unknown primary carcinoma. In contrast to previously reported cases, the patient had no gastrointestinal or systemic symptoms. Manifestation of metastatic disease process as mediastinal LNs was also unusual. Endoscopic ultrasonography-guided fine needle aspiration enabled the diagnosis of mucinous adenocarcinoma, leading to discovery of the primary gastric malignancy.