Non-Hodgkin lymphoma may occasionally contain large transformed cells resembling Hodgkin and Reed-Sternberg cells (HRS cells). We report a 63-year-old man with HRS cells in a recurrent mantle cell lymphoma (MCL). The patient initially presented with orbital MCL and recurred after 8 years with widespread involvement. The HRS cells were present in the recurrent disease but not in the initial orbital lesions, suggesting a transformed event after a prolonged disease course. Morphologically, the HRS cells were single cells and small clusters among the MCL cells and were frequently accompanied by histiocytes but without eosinophils or other inflammatory cells. The HRS cells showed a phenotype of classic Hodgkin lymphoma (cHL). The HRS cells were clonally related to the MCL, which was demonstrated by the presence of identical t(11;14) that resulted in productive cyclin D1 expression in both cell types. Review of the literature identified 7 additional MCL cases that showed a spectrum of clinical and pathologic features ranging from scattered HRS cells to true composite MCL and cHL. The HRS cells were clonally related to MCL in 4 cases (including the current case) and unrelated in 2 cases. These findings suggest MCL with HRS cells is a heterogeneous group that may represent a spectrum of transformation at the various stages. Proof of clonal relationship between HRS cells and MCL is useful to distinguish these cases from true composite MCL and cHL.