Barrett's esophagus develops as a complication of regurgitant esophagitis and predisposes patients to the development of dysplasia and esophageal adenocarcinoma. Prior ultrastructural studies have suggested that Barrett's epithelium is a mucous secretory epithelium that shares some morphologic features with the intestine. The origin and development of Barrett's epithelium and the cellular abnormalities accompanying its neoplastic progression are poorly understood. In an attempt to better understand the histogenesis of the mucus-producing cells that predominate in Barrett's epithelium, these cells were studied by transmission electron microscopy and compared with other upper gastrointestinal epithelia: esophageal glands, normal gastric surface, pit, and cardiac gland regions, gastric intestinal metaplasia, and normal jejunal villous tip and crypt regions. A total of 134 mucosal biopsies from the stomach and esophagus of 28 patients with Barrett's esophagus and 37 biopsies from 14 other control patients were studied. Barrett's specialized metaplastic surface cells display a spectrum of ultrastructural features among three main surface columnar epithelial cell types: mucous cells resembling those seen in the normal gastric surface epithelium or resembling mucous neck cells normally seen in the gastric pits; goblet cells similar to those seen in the jejunum; and "pseudoabsorptive" cells with features of both gastric mucous secretory cells and jejunal absorptive cells. Cytoplasmic organelles of Barrett's specialized metaplastic, normal gastric mucous neck, and normal gastric surface mucous epithelial cells, including rough endoplasmic reticulum, glycogen aggregates, Golgi apparatus, and mucous secretory granules, have common ultrastructural features associated with mucus synthesis. The morphologic heterogeneity of Barrett's specialized metaplastic cells and common ultrastructural features associated with normal mucus biosynthesis suggest that they develop from a gastrointestinal stem cell that retains the capacity for a wide range of normal and abnormal differentiation in the esophagus. The identity of this undifferentiated cell, which may reside in normal proximal gastric or esophageal mucosa, remains unknown. However, the gastric mucous neck cell has properties that suggest it could be the progenitor cell for Barrett's esophagus because it is a stem cell that has ultrastructural similarities to Barrett's specialized metaplastic epithelial cells and it is located in intact gastric mucosa adjacent to where Barrett's esophagus forms.