Most gastroduodenal ulcer disease results from a weakness in the normal gastric mucous barrier against the penetration of acid secreted by the stomach. Based on meticulous and insightful research, the distinguished physiologist Franklin Hollander hypothesized that the stomach is protected against its own acid secretion by a dynamic two-component mucus-mucosal barrier. Hollander and his co-workers defined the physical and chemical characteristics of the mucus components of this barrier, as well as the defense provided by the surface epithelial cell layer, which he viewed as the second line of defense (the second component). Barrier investigators at Mount Sinai demonstrated the effects of impairment of barrier function with resultant increased back-diffusion of acid, and they defined the consequences of this acid penetration into the gastric epithelium. The contribution of these workers included important observations on the natural impermeability of the gastric corpus and fundus as well as the normally increased permeability of the antrum. They also presented evidence on the role of bile in duodenogastric reflux in gastric ulcer disease and the presence of impaired barrier function in patients with gastric ulcer and pernicious anemia. Further studies included demonstration that stress and carcinogens could disrupt the gastric mucosal barrier. Disruption of the barrier, in turn, was shown to allow carcinogenesis to occur by permitting the absorption of certain carcinogens which otherwise are warded off by the barrier. The Hollander two-component gastric mucosal barrier hypothesis has, in recent years, been increasingly validated by experimental data coming from other laboratories.