Primary gastric non-Hodgkin lymphoma of mucosa-associated lymphoid tissue (MALT-NHL) is strongly related to Helicobacter pylori (H pylori) infection. CagA positive H pylori strains appear to be more aggressive, as reflected by a greater degree of gastric inflammation and higher levels of Il-8 production. Using a polymerase chain reaction-based assay, cagA status was determined in H pylori strains of 12 patients with gastric MALT-NHL, 38 patients with peptic ulcer disease, and 39 patients with non-ulcer dyspepsia. CagA-positive H pylori strains were significantly more frequent in peptic ulcer disease than in non-ulcer dyspepsia (Fisher's exact test p <0.001). An increased frequency of CagA-positive strains was not identified in gastric MALT-NHL compared to non-ulcer dyspepsia patients (Fisher's exact test P <.3). Therefore, it can be concluded that infection with more aggressive strains is associated with the development of peptic ulcer disease, but plays a minor role, if any, in the development of gastric MALT-NHL.