Histiocytic sarcoma (HS) is an extremely rare malignant neoplasm showing morphologic and immunophenotypic evidence of histiocytic differentiation. The vast majority of previously reported HSs are now generally recognized to be misdiagnosed examples of non-Hodgkin lymphomas, predominantly diffuse large B-cell lymphoma or anaplastic large cell lymphoma. The recognition of such tumors parallels the development and widespread use of immunohistochemical techniques, along with the development of molecular genetic methods to detect immunoglobulin (IG) or T-cell receptor (TCR) gene rearrangement. The 2001 World Health Organization (WHO) definition of HS requires the absence of clonal B/T-cell receptor gene rearrangements. However, the 2008 WHO classification no longer strictly requires the absence of clonal immunoglobulin heavy chain (IGH) or TCR gene rearrangement for the diagnosis of HS. Recent studies demonstrated that HSs that occur subsequent to or concurrent with B- or T-lymphoblastic lymphoma/leukemia or mature B-cell neoplasms generally show clonal IgH and/or TCR gene rearrangement. These findings suggest the possibility of transdifferentiation of the two otherwise morphologically and immunohistochemically distinctive neoplasms. In addition, a recent study suggested clonal IG gene rearrangements may be detected at a high frequency in sporadic HS, indicating that a large subset of sporadic HSs may inherit the B-lymphocyte genotype. These findings provide new insights into the pathogenesis of HS, although the etiology of HS is still unknown. HS is a diagnosis of exclusion. It is necessary to rule out other diseases that could be misdiagnosed as HS with extensive immunophenotypical analysis before diagnosing HS.