Recently, a close relation has been found between infection of the gastric mucosa by Campylobacter pylori and chronic gastritis. To establish the possible existence of characteristic morphologic changes in this disease, which can be differentiated from other unrelated forms of gastritis, we analyzed the antral biopsies obtained from 75 patients, 35 with duodenal peptic ulcer and 40 with nonulcerous dyspepsia. The diagnosis of C. pylori infection is based on positive biopsy culture or, if not, when following three requirements are met: positive urease test before 24 hours, identification of the germ by Gram stain and visualization in the tissue of microorganisms with morphology similar to that of C. pylori. We found that 85.5% of the 55 patients with C. pylori infection present active chronic gastritis with lymphoid nodes (GCA + NL), while this morphology is only found in 5 of the 20 uninfected patients. The association of GCA + NL with C. pylori infection is highly significant (p less than 0.0001). We think that it could be a local immunologic response to the stimulus of the bacterial antigen, and that it has sufficient morphologic entity to differentiate it from other inflammatory processes of the gastric mucosa of still unknown etiology.