A 30-year-old woman presented with a breast mass, and a biopsy was performed. Flow cytometry detected a prdominant monotypic B-cell population positive for CD20, CD19, and kappa while negative for CD5, CD10, and CD23 (panels A and B). Histopathology revealed a nodular and diffuse lymphoid proliferation infiltrating breast parenchyma. The lymphocytes were predominantly small with round to mildly irregular nuclei, clumped chromatin, and scant to moderate amounts of cytoplasm (panel C; hematoxylin and eosin stain; original magnification ×100 [main image] and ×600 [upper inset]). Some areas had a monocytoid appearance. These preliminary findings were suggestive of extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma). However, a cyclin D1 immunohistochemical stain showed diffuse positivity (panel C middle inset; original mangification ×600). A SOX11 stain was also positive (panel C lower inset; original mangification ×600), and CCND1/IGH rearrangement was detected by fluorescence in situ hybridization (panel D), confirming the diagnosis of CD5 − mantle cell lymphoma (MCL).This is an unusual case of CD5 − MCL involving the breast of a young patient. It provides an important reminder that thorough immunophenotyping is required for all new lymphoma diagnoses. Specifically, cyclin D1 should be evaluated regardless of CD5 expression. This case could have easily been misdiagnosed as MALT lymphoma, as it was the most likely possibility given the clinical presentation, morphology, and immunophenotype by flow cytometry.