A 76-year-old man with hepatocellular carcinoma (HCC) was admitted to our hospital suffering from rapidly progressing dyspnea. Chest computed tomography on admission merely showed ground-glass patterns in both lung fields without thrombi in the pulmonary trunk. On the third day, pulmonary blood flow scintigraphy was performed because of progression of his dyspnea, and showed multiple defects indicating widespread thrombi in the peripheral pulmonary arteries. He died of respiratory failure on day 13. A needle necropsy revealed the presence of multiple foci of adenocarcinoma nests in the lungs, suggesting venous thrombi from the poorly differentiated HCC. Although HCC frequently metastasizes to the lung, patients with lung metastasis rarely result in respiratory failure. It is well known that some patients with adenocarcinoma including HCC can develop respiratory failure owing to pulmonary tumor thrombotic microangiopathy (PTTM). In our case, however, pathological examination showed widespread tumor microemboli in the lung, but no stenosis or fibrocellular intimal proliferation in the small arteries and arterioles, which are essential findings of PTTM. Although we concluded that the respiratory failure in this case was mainly caused by widespread tumor microemboli, it remains unclear why such dissemination rapidly developed.