To determine the clinical importance of Campylobacter pyloridis infection, its association with gastric inflammation, and the response to drug therapy, patients with a duodenal or gastric ulcer (n = 63), patients with nonulcer dyspepsia (n = 240), and asymptomatic volunteers (n = 34) were studied. In a prospective longitudinal study, the type, intensity, and distribution of inflammation in antral biopsy specimens were correlated with the presence of C. pyloridis. Campylobacter pyloridis was cultured from antral biopsy specimens in 98% of the ulcer patients, 70% of the nonulcer dyspepsia patients, and 20% of the asymptomatic volunteers. The dependency of chronic active gastritis on the presence of C. pyloridis was shown by an association of gastritis with positive culture and healing of gastritis with negative culture after various therapeutic regimens. Spontaneous disappearance of C. pyloridis never occurred. Colloidal bismuth subcitrate, amoxicillin, and the combination of colloidal bismuth subcitrate and amoxicillin were effective therapies in eradicating C. pyloridis. Recolonization with the same bacterial subtype and recurrence of gastritis frequently occurred within 1 mo after initial eradication. In this study we demonstrate ultimate normalization of gastric mucosa after successful eradication of C. pyloridis. Especially complete normalization of gastric mucosa after amoxicillin monotherapy provides additional strong evidence for a true cause-effect relationship between C. pyloridis colonization and gastritis.