Helicobacter pylori is the major cause of chronic gastritis worldwide. With an estimated rate of infection of over one half of the world's population, it is responsible for extensive morbidity and mortality. Infection with this organism does not appear to spontaneously resolve. Instead it reaches a chronic stage from which a number of outcomes are possible. This article reviews those outcomes that have been linked to H. pylori and explores the pathogenesis while attempting to resolve the discrepant paths infection can take. The associations include duodenal and gastric ulcers and the majority of gastric lymphomas of B-cell type derived from the mucosa-associated lymphoid tissue (MALT). Chronic gastritis has also been shown to evolve into atrophy with intestinal metaplasia in certain populations. This change in the gastric epithelium has been linked with an increased risk of gastric adenocarcinoma. Microsc. Res. Tech. 48:313-320, 2000. Published 2000 Wiley-Liss, Inc.