A previously healthy 50-year-old man who was a smoker was referred to the emergency department with progressive dry cough and dyspnea for 1 month. He had been prescribed antiallergic and antitussive medications by a family doctor; however, they were ineffective. The patient had no family history of cardiopulmonary disease or malignancy. Physical examination and blood tests in the emergency department were non-diagnostic. Electrocardiography revealed negative T waves, and echocardiography was suggestive of pulmonary hypertension. Enhanced computed tomography (CT) revealed no apparent pulmonary embolism. He was prescribed a bronchodilator and corticosteroid under a diagnosis of cor pulmonale due to chronic obstructive pulmonary disease; however, his symptoms did not alleviate. The patient was admitted to our hospital two days later. His vital signs on admission were as follows: oxygen saturation, 94%; pulse rate, 100 beats per minute; body temperature, 37.1 °C; blood pressure, 130/93 mmHg; and respiratory rate, 18 breaths per minute. Physical examination revealed right cervical lymphadenopathy, with the affected lymph node measuring 10 mm in size. Laboratory findings were as follows: 95.7 (normal: < 18.4) pg/mL of brain natriuretic peptide, 6.9 (normal: < 1.0) µg/mL of D-dimer, 134 (normal: 124–222) U/L of lactate dehydrogenase, 1.77 (normal: < 0.15) mg/dL of C-reactive protein, 8.7 (normal: < 5.0) ng/mL of carcinoembryonic antigen, and 19.4 (normal: < 2.1) ng/mL of cytokeratin-19 fragment. Chest radiography revealed bilateral hilar lymphadenopathy. Enhanced CT showed no pulmonary embolism or deep vein thrombosis; however, right cervical, bilateral hilar/mediastinal, and upper abdominal lymphadenopathy was observed. Moreover, septal thickening and ground-glass opacities were predominantly detected in the lower lung lobes. Electrocardiography showed negative T waves in precordial leads, and echocardiography indicated pulmonary hypertension with normal left ventricular function. We suspected PTTM because of the subacute course, pulmonary hypertension without apparent etiology, elevated tumor marker levels, and diffuse lymphadenopathy. The primary cancer origin could not be determined using CT or tumor markers. Echo-guided needle biopsy of the right cervical lymph node was performed on day 1. Stamp cytology of the biopsy revealed atypical cells of epithelial origin. On day 2, a transbronchial lung biopsy was performed. On day 3, an interim report regarding the pathological findings of cervical lymph node biopsy revealed adenocarcinoma with signet ring cell carcinoma (SRCC). Therefore, gastric cancer was suspected; however, upper endoscopy performed by a trained gastrointestinal endoscopist showed no mucosal abnormalities and a fully distensible gastric wall. Colonoscopy results were also normal. On day 4, the patient started receiving edoxaban (60 mg/day) for probable hypercoagulability due to PTTM. On day 5, the reported results of cervical lymph node biopsy with immunohistochemical staining were consistent with those of gastric cancer. The pathological findings of transbronchial lung biopsy were unremarkable. At this time, the patient’s status was stable without any apparent changes. On day 6, his dyspnea mildly progressed, and oxygen delivery was initiated at 2 L/min via a nasal cannula. On day 7, after consultation with gastroenterologists at our institution, administration of imatinib (200 mg/day), prednisolone (30 mg/day), and tegafur/gimeracil/oteracil plus oxaliplatin (SOX) therapy was initiated. On day 8, the patient’s hypoxia suddenly worsened, and he subsequently died. An autopsy was performed, and macroscopic examination showed no abnormalities in the gastric wall (Additional file a); however, lymphadenopathy surrounding the stomach was detected. Macroscopic pulmonary thromboembolism was not observed. Multiple serial sections of the whole stomach were prepared. Microscopically, two gastric cancer lesions of pure SRCC that measured 15 12 mm and 8 8 mm were identified in the pyloric region. Although these lesions were within the lamina propria, multiple lymphovascular invasions separate from the two superficial lesions were observed in the layers from the submucosa to the subserosa. Multiple lymph node metastases surrounded the stomach. No findings of Helicobacter pylori-associated gastritis were observed. Moreover, tumor emboli and thickened epithelial cells of the small pulmonary arteries were consistent with PTTM. Carcinomatous lymphangiomatosis was also observed. Examination of organs other than those mentioned above showed no tumor cell in the viscera or vascular system. The final diagnosis was PTTM in occult early gastric cancer that was undetectable on upper endoscopy.