The evidence that H. pylori causes gastritis in humans comes from both primary and secondary observations. The most important primary observations are the human volunteer studies, the animal models, and the treatment studies with antimicrobial agents. Supporting information comes from studies showing the specific association of H. pylori infection with type B gastritis and with gastric (but not intestinal) epithelial cells; the specific ultrastructural lesions, including adherence pedestals; the ubiquity and stability of the immune response; the response to bismuth treatment; and the association with epidemic gastritis and hypochlorhydria. It is important to note that all of Koch's postulates have been fulfilled, and despite nearly universal initial skepticism, no evidence exists against the hypothesis that H. pylori plays an etiologic role in type B gastritis. Therefore, it is reasonable to conclude that H. pylori is a pathogen in humans. The known features of H. pylori infection are listed in. Infection is chronic and common throughout the world, with a higher prevalence in developing countries than in developed countries. The prevalence of H. pylori infection increases with age in parallel with that of gastritis. Acquisition of H. pylori infection does not appear to have any seasonality, and infection is equally common among men and women. Without a significant animal or environmental reservoir for human strains of H. pylori, person-to-person contact appears to be the most likely mode of transmission. Exactly how the organism is transmitted from the stomach of one person to that of another remains unclear. Also unknown are the factors which determine who becomes ill after infection; why one person has gastritis alone while another person develops a duodenal ulcer; and how the traditional risk factors for ulcer disease, such as smoking, aspirin, and alcohol, interact with H. pylori infection. Finally, the long term neoplastic consequences of infection must be understood. Further elucidation of the natural history of H. pylori and the consequences of H. pylori infection is the most important goal for future study.