A74-year-old male with a history of a tonsillar lymphoma developed diarrhea. Investigations led to detection of extensive intestinal lymphomatous polyposis (mantle cell lymphoma). After an aggressive clinical course with associated nodal and peripheral blood involvement, death followed within three months. Postmortem studies revealed widespread dissemination within the entire gastrointestinal tract, including the esophagus, stomach, and small and large intestines. Although this type of lymphoma is rare and accounts for only about 1% to 8% of all forms of primary B cell gastrointestinal lymphomas in North America, separation from other subtypes has become more important because of reported responses of mucosa-associated lymphoid tissue-lymphomas to antibiotics aimed at Helicobacter pylori eradication.