A 57-year-old man presented with generalized lymphadenopathy. Lymph node biopsy showed a vaguely nodular infiltrate of pleomorphic intermediate-sized lymphocytes (panels A and B, original magnification ×20 and ×500, respectively; hematoxylin and eosin [H&E]–stained lymph node sections) that were CD20 + (panel C, original magnification ×200), CD5 + (panel D, original magnification ×200), CD23 −, CD38 +, SOX11 + (panel H, original magnification ×200), and cyclin D1 − (panel I, original magnification ×200). Cytogenetic analysis revealed an aneuploid karyotype and an absence of t(11;14)(q13;q32). IGH or CCND1 translocations were not detected by fluorescence in situ hybridization analysis. These findings were diagnostic of a cyclin D1 − mantle cell lymphoma (MCL), a rare type of MCL often associated with upregulation of cyclin D2 or D3 (approximately 50% of cases harbor CCND2 translocations, and rare cases with CCND3 or MYCN translocations have been reported). Of note, approximately 50% of the neoplastic B cells showed cytoplasmic CD3ε chain expression by immunohistochemistry (panel E, original magnification ×200; and panel F, CD3-brown and CD20-red dual stain, original magnification ×500). Flow cytometry also detected 1% surface CD3ε + B cells. Besides CD3 and CD5, no other T-cell antigens were expressed (panel G, CD2 highlights a few scattered T cells; original magnification ×200). Polymerase chain reaction analyses for IGH and TRB gene rearrangement showed clonal and polyclonal products, respectively.Aberrant T-cell antigen expression has been described in a variety of B-cell non-Hodgkin lymphomas; however, this phenomenon is uncommon in MCL. Rare cases of MCL with aberrant CD8 or CD7 expression have been reported, but CD3ε expression has not been described. The prognostic significance of T-cell antigen expression in MCL is not known. The patient was treated in a phase 1 study (ibrutinib and obinutuzumab), with response by positron emission tomography–computed tomography imaging.