Gastric morphology, function, and immunology was studied in 68 patients with pernicious anemia (PA), 183 of their first-degree relatives, and 354 control subjects. The PA relatives and controls were comparable in age and sex distribution. In both groups, mean gastric acid output decreased and mean fasting serum gastrin levels and the prevalence of atrophic gastritis increased with age. The total prevalence of chronic gastritis was similar in the two groups, but severe atrophic gastritis of the body of the stomach (AGB), achlorhydria, parietal cell antibodies, and a raised fasting serum gastrin level were significantly more common in PA relatives than in controls. Of the PA relatives 23 had severe AGB which was indistinguishable from the gastric mucosal lesion found in PA probands and was, as a rule, accompanied by several other characteristics of type A gastritis. These included a normal antrum (78%), slight or absent inflammatory cell infiltration in the gastric mucosa (70%), achlorhydria (91%), high fasting serum gastrin level (83%), parietal cell antibodies (65%), and intrinsic factor antibodies (22%). The mean age and the proportion of subjects with slight and moderate AGB of all AGB subjects was significantly lower in PA relatives than in controls. This suggests an early onset and a rapid progression from mild to severe AGB in PA relatives. Thus, the PA relatives appear to consist of two populations, one with a high and one with little or no proneness to severe AGB. This bimodal distribution suggests the participation of a single major factor, probably genetic, in the pathogenesis of severe AGB in PA relatives.