Primary gastrointestinal lymphoma comprises a group of distinctive clinicopathological entities, most of which are not included in current lymph node-based lymphoma classifications. They may be 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. These 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 influenced 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; high grade tumours also may arise de novo and these probably also belong to the MALT group. Immunoproliferative small intestinal disease (IPSID) is a special form of MALT lymphoma with a restricted geographic distribution, which is characterized by synthesis of alpha heavy-chain immunoglobulin. Other gastrointestinal B-cell lymphomas include mantle cell lymphoma, which presents as lymphomatous polyposis, and Burkitt's or Burkitt-like lymphoma. 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, which is increased in celiac disease.