Histiocytic sarcoma is a rare, malignant neoplasm of the lymphohematopoietic system that usually occurs in the skin, lymph node, and intestinal tract. Here we describe a unique case of primary central nervous system histiocytic sarcoma that initially showed an indolent clinical course following local resection and radiotherapy. However, relapse of disease within the mediastinum was noted 3 1/2 years later. Biopsies of the initial brain lesion and subsequent mediastinal recurrence each revealed an identical, diffuse proliferation of histiocytes with expression of CD45, CD68, and CD163 but not pan-cytokeratin, epithelial membrane antigen, CD3, CD15, CD20, CD30, CD43, CD79a, CD138, myeloperoxidase, ALK-1, PAX-5, CAM 5.2, S100, CD1a, or glial fibrillary acidic protein. In the literature, central nervous system histiocytic sarcoma portends a poor prognosis with median survival of 4.5 months. To our knowledge, this case represents the first case of "low-grade" primary central nervous system histiocytic sarcoma with relatively indolent clinical course. A thorough discussion of the differential diagnosis of histiocytic sarcoma and a review of primary central nervous system histiocytic sarcoma are also presented.