Although lymphoid tissue is absent in normal gastric mucosa, primary lymphomas arise in the stomach and most of these recapitulate the features of mucosa-associated lymphoid tissue (MALT). Gastric lymphoid tissue is known to be acquired in response to local infection by Helicobacter pylori, and we have confirmed this in 450 patients with H pylori-associated gastritis of whom 125 showed mucosal lymphoid follicles. In 8 patients, B lymphocytes infiltrated epithelium, which is a feature characteristic of MALT. We also examined 110 cases of gastric MALT lymphoma and found H pylori infection in 101 of these (92%). We conclude that gastric MALT is acquired in H pylori infection and that this provides the necessary background in which MALT lymphoma might develop.