We report a patient with primary histiocytic sarcoma of the spleen associated with prominent hemophagocytosis. Although thrombocytopenia, probably due to hemophagocytosis, was refractory to corticosteroid therapy, the transfusion of platelets, and splenic irradiation, partial splenic embolization was effective and facilitated splenectomy for a diagnosis. The majority of the spleen showed necrosis, but viable neoplastic cells with pleomorphic nuclei and abundant cytoplasm, showing occasional erythrocytes or leukocytes, were still discernible. The neoplastic cells expressed CD68, lysozyme, and S-100 protein, and were negative for lymphoid, myeloid, and epithelial cell markers. CD163, a monocyte/macrophage-specific molecule, was positive in only some of them. Despite multiagent chemotherapy, the patient died of the disease, showing a rapidly progressive clinical course. Although the preoperative diagnosis of primary splenic histiocytic sarcoma is difficult, it has been confirmed in patients with splenomegaly of unknown etiology that clinicolaboratory features suggestive of hemophagocytosis may be important clues suggestive to the disease. CD163 expression by neoplastic cells could be confirmed only after careful observation, because the molecule may only be seen in some of the neoplastic cells.