Collagenous gastritis is rare; there are only four previous case reports. Histologic features seem to overlap with the other "collagenous enterocolitides"; however, pathologic criteria are not yet established for the diagnosis of collagenous gastritis. We describe an additional case of ostensible collagenous gastritis in a patient who initially presented with celiac sprue and subsequently developed colonic manifestations of mucosal ulcerative colitis. Endoscopic biopsies of the stomach revealed deposition of patchy, very thick bandlike subepithelial collagen in gastric antral mucosa, focal superficial epithelial degeneration, numerous intraepithelial lymphocytes, and a dense lamina propria lymphoplasmacytic infiltrate. Image analysis evaluation of gastric antral biopsies demonstrated a mean thickness of subepithelial collagen of 27.07 micron. Morphologic comparison was made with age-matched control groups of 10 patients who had normal gastric mucosal biopsies and 10 patients who had "chronic" gastritis, which revealed mean subepithelial collagen measures of 1.37 micron and 1.19 micron, respectively. We compared these morphologic findings with those of all previous case reports of collagenous gastritis and propose a pathologic definition based on the limited combined data. It seems that subepithelial collagen is dramatically thickened in reported cases of collagenous gastritis, with a cumulative mean measure of 36.9 micron. It is also apparent from this and previous reports that the thickened subepithelial collagen is accompanied by a chronic or chronic active gastritis and sometimes intraepithelial lymphocytes and surface epithelial damage. Recently described associations of lymphocytic gastritis, sprue, and lymphocytic colitis as well as collagenous and lymphocytic colitis suggest a common pathogenesis that empirically may include collagenous gastritis in the same disease spectrum. We propose that collagenous gastritis can be confidently identified by using analogous defined features of collagenous colitis: subepithelial collagen more than 10 micron in a patchy distribution, lamina propria lymphoplasmacytic infiltrates, intraepithelial lymphocytes, and surface epithelial damage. Collagenous gastritis also seems to have the same spectrum of associated clinical findings as collagenous colitis, including frequent coexistence of celiac sprue, watery diarrhea syndrome, and female predominance.