There have been a number of previous case reports of lymphomas in the form of multiple mucosal polyps affecting variable lengths of colonic and intestinal mucosa. Invariably these have been classified histologically as mantle cell lymphomas. We report a case of this rare syndrome that illustrates many of its typical features but which demonstrates significant histological and immunophenotypic differences. A 67-yr-old man was referred with a 3-month history of altered bowel habit and a barium enema suggestive of extensive ulcerative colitis. At colonoscopy, diffuse umbilicated mucosal polyps were seen throughout the colon and a larger circumferential mass lesion at the ileocecal valve. Biopsies demonstrated a diffuse B-cell lymphoma consistent with "multiple lymphomatous polyposis" of the colon. Flow cytometry revealed CD25 positive/CD5 negative lymphoid cells confirming marginal B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) type. Upper gastrointestinal investigations confirmed similar involvement of the stomach, duodenum, and small bowel. Helicobacter pylori was absent. Flow cytometry abnormalities consistent with marrow involvement were present. These features led to a diagnosis of stage IV marginal zone B-cell lymphoma of MALT type, presenting as multiple lymphomatous polyposis of the gastrointestinal tract. Three months of combination chemotherapy resulted in an excellent symptomatic and endoscopic response.