Mantle cell lymphoma presenting as lymphomatoid polyposis coli in a 65 y/o male. The patient presented with diarrhea, bleeding per rectum and weight loss. Colonoscopy showed multiple polyps in the colon. Colonic biopsy (shown here) revealed diffuse sheets and nodular aggregates of medium-sized atypical lymphoid cells with irregular, indented nuclei and punctate nucleoli. A few larger atypical lymphoid cells with more dispersed chromatin and prominent nucleoli were also present. Mitotic activity was increased. The immunohistochemical profile of the tumor cells was as follows: POSITIVE: CD20, Cyclin D1, CD5 (weak), and bcl-2. NEGATIVE: CD10, CD23, CD3, CD56, and bcl-6. Ki67 proliferation index was about 70%. Cytogenetics showed t(11;14)(q13;q32) translocation. Subsequent work-up showed involvement of bone marrow and multiple abdominal and mediastinal lymph nodes.