Dysplasia is one of the most important subjects regarding carcinogenesis of the esophagus, because there is continuing controversy as to whether esophageal dysplasia is a cancerous lesion or a noncancerous lesion. In this study, it is histopathologically shown that dysplasia has a close correlation with cancer itself and that there is no substantial difference in the cell proliferative activity of dysplasia and intraepithelial carcinoma. These findings thus show that dysplasia has as high a potential for malignancy, thus it should be treated as an extremely early cancerous lesion of the esophagus to substantially improve the long term results of this disease. Conversely, esophageal carcinoma with glandular and/or mucus-secreting components is commonly found in addition to the ordinary component of squamous cell carcinoma, which indicates that this type of esophageal tumor originates not only from the squamous epithelium but also from the esophageal mucus gland or the ductal epithelium. These findings support the concept of field carcinogenesis in esophageal carcinoma.