Eighteen autopsied cases of a clinical syndrome designated as “Thailand epidemic hemorrhagic fever” were reviewed. Clinical observations, such as age, sex, and symptoms, were similar in all cases. Pathologically, changes were found which were common to almost all of the patients and which were distinctly characteristic. The main changes were vascular alterations, such as congestion and vasodilation, edema and hemorrhage, and cellular proliferation of reticuloendothelial tissue. Interventricular subendocardial hemorrhage of the left side of the heart was striking. Necrosis was conspicuously absent in various organs. It is concluded that “Thailand epidemic hemorrhagic fever” is to be considered as a clinicopathological entity, probably caused by certain arthropod-borne viruses.