Gastrointestinal lymphomas comprise a group of distinctive clinicopathological entities of B- or T-cell type, with primary gastrointestinal Hodgkin's disease being extremely uncommon. Most low-grade B-cell gastrointestinal lymphomas are of mucosa-associated lymphoid tissue (MALT) type, so called because they recapitulate the features of MALT rather than those of lymph nodes. Paradoxically, however, most MALT lymphomas arise in the stomach, which normally contains no organized lymphoid tissue. Gastric MALT lymphomas appear to arise in MALT acquired as a reaction to infection of the stomach by Helicobacter pylori and their growth can be inhibited by eradication of this organism from the stomach. Low-grade MALT lymphomas, which usually have a very favorable clinical course, may undergo high-grade transformation but high-grade diffuse large B-cell lymphomas may also arise de novo. Immunoproliferative small intestinal disease (IPSID) is a special form of MALT lymphoma characterized by synthesis of alpha heavy-chain immunoglobulin and a restricted geographic distribution. Other B-cell lymphomas that tend to arise in the gastrointestinal tract include mantle cell lymphoma, which presents as lymphomatous polyposis, Burkitt's lymphoma, and B-cell lymphomas associated with immunodeficiency states. Enteropathy (celiac disease)-associated T-cell lymphoma (EATL) is the most common primary gastrointestinal T-cell lymphoma This is a clinically aggressive tumor that arises from the intraepithelial T-cell population.