The frequency of ducts of esophageal glands proper and Paneth cells was studied in endoscopic biopsy specimens obtained from 49 patients diagnosed as having Barrett's esophagus. Esophageal gland ducts were found in 5 (10%) of the subjects. Paneth cells were observed in 15 (31%) of the 49 patients, being located mainly among mucous cells of intestinalized crypts although a few were also noted in the surface epithelium. Since the presence of esophageal gland ducts in the columnar epithelium-lined mucosa is a pathognomonic feature of Barrett's esophagus, 10% of the patients reviewed were considered to have an unequivocal diagnosis of Barrett's esophagus. Barrett's mucosa showed histological features similar to incomplete-type intestinal metaplasia of gastric mucosa, except for the presence of Paneth cells.