It is now generally accepted that Campylobacter pylori is closely associated with peptic ulcer disease and chronic type B gastritis. Whether C. pylori is the direct etiologic cause of either or both of these illnesses remains unclear. Possible pathophysiologic effects of C. pylori are still a matter of debate and conjecture. Utilizing a small group of patients with gastric ulcers and chronic gastritis, we examined the ultrastructural relationship between C. pylori and gastric cells. Forty-eight percent of our gastric ulcer patients and 57% of our chronic gastritis patients had C. pylori in their lower corpus mucosa. Examination with the transmission electron microscope indicated a very close proximation by C. pylori to the surface epithelial cells strongly suggesting adherence. We also describe for the first time the invasion of gastric cells by C. pylori Although an uncommon occurrence, we had repeated observations of C. pylori invading surface epithelial cells, parietal cells, and chief cells. Most of the intracellular C. pylori were intact but other forms appearing to be degenerating organisms were also seen. We suggest that cell invasion may be one mechanism by which C. pylori causes pathologic changes in the gastric mucosa. These observations may also explain why C. pylori chronically infects gastric cells and frequently recurs after treatment.