To clarify the histology and morphogenesis of the double muscularis mucosae in Barrett's esophagus, eight specimens resected from patients with Barrett's esophagus were compared histopathologically with 352 specimens resected from patients without Barrett's esophagus. A double muscularis mucosae was observed in seven (87.5%) of the eight cases with Barrett's esophagus, but in none of the 352 cases without Barrett's esophagus. The mucosa in the segment of Barrett's esophagus consisted of columnar epithelium, a superficial lamina propria, a superficial muscularis mucosae, a deep lamina propria, and a deep muscularis mucosae. The distal end of the superficial muscularis mucosae was connected to the deep muscularis mucosae at the esophagogastric junction, and its proximal end was located in fibrous tissue below the squamocolumnar junction of the mucosal epithelium or the distal edge of the erosive lesion. The deep muscularis mucosae in the portion with Barrett's esophagus was continuous with the original muscularis mucosae of the proximal esophagus and muscularis mucosae of the stomach. Barrett's esophagus is considered to be not merely a metaplastic lesion within the epithelium, but a newly developed lesion containing columnar epithelium, lamina propria, and a superficial muscularis mucosae on the lamina propria of the esophageal mucosa.