Primary gastric low-grade B-cell lymphomas are neoplastic mimics of mucosa associated lymphoid tissue (MALT) as exemplified by Peyer's patches in the terminal ileum. Architectural and immunophenotypic properties of the neoplastic cells suggest that they originate from MALT-derived marginal zone B-cells. Paradoxically, the normal human stomach is devoid of organized MALT within which a lymphoma can develop. Lymphoid tissue is acquired in the stomach in response to antigenic stimulation, predominantly associated with Helicobacter pylori infection. Studies of patients with low-grade MALT lymphoma have confirmed a high incidence of H. pylori infection and suggest that the infection predates neoplastic transformation. Certain morphological features of MALT lymphomas suggest that the tumor cells remain responsive to antigen drive. Given the close association between gastric MALT lymphoma and H. pylori, it is possible that this organism provides such a drive. In vitro studies have shown that the tumor cells proliferate in a T-cell-dependent way to the presence of H. pylori. Several studies have now demonstrated that eradication of the organism in patients with low-grade gastric MALT lymphoma can result in regression of the tumor. In cases with a high-grade component, the associated low-grade part may regress, but most high-grade gastric MALT lymphomas are unresponsive to this conservative therapy.