A prominent pathological feature of murine autoimmune gastritis is a pronounced mucosal hypertrophy. Here we examined factors that may be responsible for inducing this hypertrophy. As gastrin is known to be both an inducer of gastric mucosal cell proliferation and is elevated in autoimmune gastritis, mice deficient in gastrin were thymectomised at day 3 and assessed for autoimmune gastritis. Gastrin-deficient mice showed all the characteristic features of murine autoimmune gastritis, including gastric unit hypertrophy due to hyperproliferation and accumulation of immature epithelial cells, decreases in the number of zymogenic and parietal cells and autoantibodies to the gastric H⁺K⁺ ATPase. Hence gastrin is not required for either establishment of chronic gastritis or development of the typical pathological features of this disease. We also examined mRNA levels of a number of gastric mucosal growth factors in RNA samples from mice with hypertrophic autoimmune gastritis. Members of the Reg family RegIII and RegIII were greatly elevated in mice with hypertrophic gastritis, whereas RegI and amphiregulin (an EGF receptor ligand) were more modestly and/or inconsistently induced. These data demonstrate that induction of gastric mitogenic factors, such as members of the Reg family, can be achieved in inflammatory situations by gastrin-independent pathways . Members of the Reg family, in particular RegIII and RegIII, are good candidates to be involved in inducing the mucosal hyperproliferation in autoimmune gastritis. These findings are likely to be of relevance to other gastric inflammatory conditions.