It is proposed that two types of atrophic gastritis, Type A and B, can be distinguished in man on the basis of the distribution of the disease in the stomach and results of autoimmune serologic tests. In Type A gastritis the test for parietal cell antibody (PCA) is positive and serum gastrin levels are high, reflecting an intact antral mucosa demonstrable also by biopsy. Corpus changes are diffuse and gastric secretion of acid severely impaired. In Type B gastritis the test for PCA is negative and serum gastrin levels are low, reflecting antral damage. Corpus changes tend to be focal and gastric secretion of acid is moderately to severely impaired. Chronic gastritis occurs frequently (prevalence, 28%) in Caucasian adult populations, Type B being four times more frequent than Type A. Several endocrine diseases and other organ-specific autoimmune reactions are strongly linked to Type A gastritis; pernicious anemia evolves almost exclusively from this type. Gastrointestinal symptoms, gastric hemorrhage and gastric ulcer are prominent sequelae of Type B gastritis. Iron deficiency and gastric carcinoma can develop from both forms of gastritis, but Type B is the more important precursor of carcinoma. Autoimmunity is believed to be of primary importance in Type A gastritis and is probably genetically determined. Environmental factors, including several mucosal “irritants” may determine the occurrence of Type B gastritis.