A case of diffuse lymphomatous polyposis of the gastrointestinal tract is reported. The patient presented with abdominal pain and weight loss and x-rays revealed multiple polyps involving the entire gastrointestinal tract. Biopsies confirmed the diagnosis of lymphomatous polyposis. The patient also had involvement of the bone marrow and supraclavicular lymph node. Immunologically this lymphoma was characterized as a monoclonal proliferation of B lymphocytes bearing immunoglobulin M, type kappa. Diffuse lymphomatous polyposis of the gastrointestinal tract appears to be a generalized malignancy of uncommitted B cells of Peyer's patches. The migratory properties of these cells may account for the tendency to dissemination of lymphomatous polyposis. Diffuse lymphomatous polyposis of the gastrointestinal tract is a distinct entity, separate from the diffuse gastrointestinal lymphoma known as Mediterranean-type lymphoma.