CD5 expression in neoplastic large B-cells in T-cell/histiocyte-rich large B-cell lymphoma has not been reported, to the best of our knowledge. Here we describe the first case of CD5+ T-cell/histiocyte-rich large B-cell lymphoma that is well documented by histomorphology, immunohistochemistry, flow cytometry immunophenotyping and sorting, and immunoglobulin heavy-chain gene rearrangement study by polymerase chain reaction. The expression of CD5 in large neoplastic B-cells was demonstrated by immunohistochemistry and multicolor flow cytometry. The clonal nature of the CD5+ neoplastic B-cells was confirmed by rearranged immunoglobulin heavy (IgH) chain with polymerase chain reaction (PCR) of flow cytometry-sorted CD5+/CD19+/kappa+ cells. The CD5+ neoplastic large B-cells expressed bcl-6 and MUM1/IRF4 but not CD138 by immunohistochemistry. This suggests that the neoplastic cells may be of late germinal-center B-cell/ early post-germinal center B-cell origin. The patient responded to chemotherapy, CHOP (Cytoxan, doxorubicin, vincristine, and prednisone), and Rituxan very well and is currently in complete remission clinically. We propose that the current case, CD5+ T-cell/histiocyte-rich large B-cell lymphoma, represents a variant of recently reported de novo CD5+ diffuse large B-cell lymphomas. Our patient has had an excellent response to treatment; however, the clinical and biologic significance of CD5 expression in T-cell/histiocyte-rich large B-cell lymphoma requires further studies. Awareness of the CD5+ T-cell/histiocyte-rich large B-cell lymphoma variant will prompt pathologists to perform CD5 immunohistochemical stain in cases of T-cell/histiocyte-rich large B-cell lymphoma. This will lead to identifying more cases to understand the clinical and biologic characteristics of this variant.