Follicular lymphoma (FL) often transforms to diffuse large B-cell lymphoma (DLBCL) during its protracted clinical course. Rarely, histiocytic sarcoma (HS) occurs secondary to or concurrent with FL, although the biological relationship between these 2 morphologically and immunophenotypically distinct entities in the same individual has not been well characterized. We report a unique case showing the sequential occurrence of first, HS and then DLBCL in a patient with a remote history of FL. In this case, HS developed 17 years after the diagnosis of FL and was shown to partly retain the immunophenotypic features characteristic of FL, while showing the morphologic and immunophenotypic profiles diagnostic of HS. DLBCL occurred 18.5 years after FL. Both HS and DLBCL harbored the IGH/BCL2 fusion gene, a hallmark of FL, per interphase fluorescence in situ hybridization analysis. Immunoglobulin gene rearrangement studies showed a clonal rearrangement of the IGH gene in both HS and DLBCL with identical amplicons, suggesting a shared origin of the neoplastic clones. These data support the hypothesis of transdifferentiation or transevolution in a mature B-cell neoplasm, and, in addition, suggest the possibility of a divergent (bilineal) neoplastic transformation of FL in a single individual.