An 80-year-old man underwent sigmoidectomy for adenocarcinoma. Six months later, after a near-syncope incident, pancytopenia was detected in the absence of occult blood in the stools. A bone marrow biopsy showed malignant lymphoma, suggestive of mantle cell lymphoma (MCL). Colonoscopy at this time revealed 3 colonic tubular adenomas. Reassessment of the histology of the colonic polyps and appropriate immunohistochemical stains showed that the lamina propria of one of the tubular adenomas was infiltrated by MCL. Reexamination of the sections taken at the time of the original sigmoidectomy showed MCL in 2 of the regional lymph nodes removed at that time, but no evidence of lymphoma in the colon was found. To our knowledge, this is the fifth reported case of synchronous occurrence of intestinal MCL and colonic carcinoma and the first report of MCL presenting in a tubular adenoma of the colon.