Esophageal carcinoma simulated Paget's disease in a 60-year-old Japanese man with 3 months of difficulty swallowing and pain. Results of an esophageal biopsy indicated undifferentiated carcinoma, but after esophagectomy and partial gastrectomy, the resected esophagus showed that the mucosa was diffusely indurated with irregular, reticulated erosions. There was no tumor mass or ulcer. Histologic examination showed an extensive intraepithelial growth of cancer cells without any glandular or squamous cell differentiation. Tumor cells were large and round and contained large nucleoli and ample, clear, or pale-staining cytoplasm, similar to the cells of Paget's disease. Tumor cells multifocally invaded into the mucosal and submucosal ducts where they formed papillary and tubular nests, indicating adenocarcinomatous differentiation, but there was no invasive growth beyond the basement membrane. Tumor cells in the epithelial layer were negative for periodic acid Schiff (PAS) and alcian blue, but partly positive for epithelial membrane antigen (EMA) and CEA, whereas those in the submucosal ducts and glands were strongly positive for PAS, alcian blue, carcinoembryonic antigen, and EMA, especially at their luminal surfaces. Tumor cells were negative for S-100, neuron-specific enolase, and melanin. These findings indicate that the tumor in the present case was a primary esophageal carcinoma with partial adenocarcinomatous differentiation, showing an extensive intraepithelial Pagetoid growth of its undifferentiated component. This is the first case of esophageal Paget's disease presenting as intraepithelial growth alone.