Campylobacter pylori is a newly described, spiral-shaped, gram-negative bacillus that is oxidase positive, catalase positive, and urease positive and grows slowly in culture. Although observed in human tissue at the beginning of the century, it was not cultured until 1982. Because there are significant morphological and genetic differences between this organism and other species of Campylobacter, it will probably be reclassified in a new genus. Current information indicates that the organism primarily resides in the stomach tissue of humans and nonhuman primates and may occasionally spread to the esophagus or other parts of the alimentary tract under appropriate conditions. Significant evidence has accumulated in the last several years to show that it causes gastritis, and there is mounting evidence that it may participate in the development of duodenal ulcers. It may also be associated with gastric ulcers and nonulcer dyspepsia. It can be detected in patients by culture of biopsy specimens or histological staining of biopsy tissue. Indirect evidence for the presence of the organism can be obtained by detection of urease in a tissue biopsy specimen, by urea breath tests, or by detection of specific antibody. It may not be necessary to implement these procedures for routine use, however, until the role of the organism can be defined better. Ultimately, the discovery of this organism may lead to radical changes in the diagnosis and treatment of gastric disease.