This report reviews the literature pertaining to collagenous gastritis and describes the clinicopathologic evolution of this disease in a patient during a 12-year period. We examined 109 biopsy specimens of gastric mucosa from 19 different endoscopic procedures for the severity and distribution of collagenous gastritis in a single patient. Assessments were undertaken for the presence of endocrine and gastrin cell hyperplasias and dysplastic epithelial changes. Relative to biopsy specimens from age- and sex-matched control subjects, the patient's biopsy specimens showed a significantly lower number of antral gastrin cells, along with a significant corpus endocrine cell hyperplasia, suggesting an increased risk of endocrine neoplasia. Gastric corpus biopsy specimens revealed an active, chronic gastritis, subepithelial collagen deposition, smooth muscle hyperplasia, and mild to moderate glandular atrophy. Additional findings of intestinal metaplasia and reactive epithelial changes indeterminate for dysplasia raise concerns about the potential for adenocarcinoma.