Collagenous gastritis, a rare disorder characterized by subepithelial fibrosis of the gastric epithelium, has previously been reported in only four patients (including three adults). We report six cases of collagenous gastritis, including five new patients. There are four females and two males. The age in years at diagnosis was 1 (1 case), 8 (2), 15 (2) and 16 (1). The presenting features included anemia (4 cases), abdominal or chest pain (4), weight loss (3), diarrhea (2), hematemesis (2), fatigue (2), and hematochezia with hypotension (1). Hypoalbuminemia was present in 2 cases. Gastric abnormalities, while apparent endoscopically in all patients, were variable, and included erythema, mucosal hemorrhages, erosions, ulcerations, exudate, granularity and nodularity, present in the gastric body and, to a lesser extent, in the antrum. In two patients the gastric mucosa endoscopically resembled variolaform gastritis. The esophagus, duodenum and ileum generally appeared normal; two patients had colitis. Histopathological abnormalities of the gastric mucosa included mononuclear infiltrates (5 cases), polymorphonuclear infiltrates (3), subepithelial fibrosis (6), fibrosis of the lamina propria (2), and smooth muscle fibers in the lamina propria (2). One patient also had collagenous colitis; another patient also had fibrosis in both duodenal and colonic mucosa. Testing for Helicobacter pylori, as well as for other bacterial pathogens and parasites, was negative in all patients. Antibody studies (parietal cell, intrinsic factor, thyroid, antinuclear, anti-mitochondrial, anti-smooth muscle, anti-neutrophilic) were negative. There were no immunoglobulin, complement or T-cell abnormalities detected. Body imaging studies were normal or demonstrated nodularity of the gastric mucosa. One patient developed diabetes mellitus; another patient developed psoriasis and achalasia. Treatment included ranitidine, omerprazole, corticosteroids, sucralfate, misoprostil, hypoallergenic diet and 5-aminosalicylates; none has induced a remission. After follow-up of 0.8 to 10 years (mean 3.8 years), all patients remained symptomatic. The etiology and pathogenesis of collagenous gastritis remains unknown. It appears to be a heterogenous chronic inflammatory process that is sometimes associated with other disorders, as well as with subepithelial fibrosis of the duodenum and/or colon.