Histiocytic sarcoma is a malignant proliferation of cells showing morphologic and immunophenotypic features similar to those of mature tissue histiocytes and is known for its rapid progression and poor prognosis. We describe a case of histiocytic sarcoma diagnosed by bone marrow biopsy. A 64-yr-old male was admitted for fever and weight loss that persisted for 8 months. The patient died undiagnosed on the 7th hospitalization day. A bone marrow biopsy performed just before the patient's death revealed diffuse proliferation of large pleomorphic neoplastic cells with large, round to oval nuclei, vesicular chromatin, and abundant foamy cytoplasm. These cells were positive for histiocytic markers, CD68, lysozyme, CD21, and S-100 protein, but negative for B-cell, T/NK-cell, and epithelial cell markers, thus confirming the presence of histiocytic sarcoma.