Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and mantle cell lymphoma usually are distinctly different in regard to clinical presentation, morphology, immunophenotype, and molecular/genetic findings. In spite of this, select cases may show overlapping characteristics and represent a diagnostic challenge. Cyclin D1 immunohistochemical staining is usually envisioned as a definitive method for resolving this differential diagnosis, with positivity supporting a diagnosis of mantle cell lymphoma. We report a case involving a 58-year-old man with a diagnosis of CLL/SLL for several years. A lymph node excision was performed after increased adenopathy was noted in the cervical region. The excised lymph node showed typical morphologic findings of CLL/SLL, including the presence of characteristic proliferation centers. Cyclin D1 staining, using 3 different antibodies, was present in scattered prolymphocytes and paraimmunoblasts, mostly within proliferation centers. Fluorescence in situ hybridization and conventional cytogenetics demonstrated trisomy 12 and an absence of t(11;14) in lymph node tissue. Focal cyclin D1 expression by immunohistochemistry in nodal CLL/SLL is quite unusual and is discussed as a potential diagnostic pitfall.