It is our hypothesis that if Helicobacter pylori could be demonstrated conclusively to have transgressed the mucosal surface into the lamina propria, this would help explain how H pylori recruits inflammatory cells. We report our immunohistochemical and electron microscopic findings that demonstrate that H pylori can be detected in the lamina propria of the stomach, offering evidence of its invasive potential. We stained 67 endoscopic gastric biopsy specimens with Warthin-Starry silver and immunoperoxidase stains for H pylori. In addition, transmission electron microscopy was performed on 1 case. The presence of surface H pylori was associated significantly with active (P < .0001) and chronic (P < .0001) inflammation. H pylori could not be identified in the lamina propria using the Warthin-Starry silver stain alone. Immunoreactivity for H pylori in the lamina propria was detected in 20 (30%) of 67 gastric biopsy specimens. Transmission electron microscopy confirmed the immunohistochemical findings. H pylori can infiltrate the lamina propria of the gastric mucosa, thereby providing morphologic evidence of its invasive capability.