Histiocytic sarcoma is an uncommon neoplasm of mature histiocytes with very poor outcome. We report an autopsy case of a true histiocytic sarcoma with characteristic symptoms of so-called "malignant histiocytosis of the intestine". The liver and spleen were enlarged, with remarkable tumor cell infiltration in the hepatic sinusoids and splenic sinuses. Tumor cells aggregated to form sporadic nodular lesions in the liver, which often showed coagulative necrosis. Infarcted lesions also occurred at the splenic subcapsular area. In addition, tumor cell infiltration was noted in the sinuses of bone marrow and lymph node. Tumor cells often demonstrated moderate pleomorphism with multinucleated giant cells. They were positive for CD68 and negative for T- and B-cell lineage markers, megakaryocytic markers, and CD30. Various examinations were done to rule out infection-associated hemophagocytic syndrome, and the absence of infectious diseases was revealed. Thus, the diagnosis of histiocytic sarcoma was made. Apart from these lesions, multiple ulcerations, some with fatal perforation, were found in the esophagus and duodenum. They showed only non-specific inflammatory changes without tumor cell involvement. The ulcers probably derived from ischemic condition through an embolic process caused by tumor cell infiltration elsewhere in the blood vessels at the periphery of the ulcers.