A 59-year-old Japanese man with Paget's disease of the esophagus is reported. The epithelium in the upper-to-middle portion of the esophagus was extensively infiltrated by Paget's cells. There was a minimal submucosal invasive area showing features of undifferentiated carcinoma. Histochemical and immunohistochemical studies showed glandular differentiation of the intraepithelial Paget's cells. Electron microscopic study demonstrated distinctive intraepithelial growth of the Paget's cells with the formation of desmosomes with nonneoplastic squamous cells, but there was no definitive evidence of glandular differentiation of the Paget's cells. These findings suggest that the entity in this case was probably adenocarcinoma in situ originating in the components of the esophageal epithelium, such as the intraepithelial or other portions of the esophageal gland ducts, with focal invasion and anaplasia. The additional control study of 154 cases of esophageal carcinoma revealed two other cases showing localized pagetoid extensions.