One hundred surgical specimens from patients with esophageal cancers were studied in detail. The characteristics of the cancers were similar to those described in the literature. Subserial sectioning of the specimens permitted determination of the type, number, and extent of noncancerous mucosal lesions and their exact location in relation to the carcinomas. Ninety-five per cent of the resected esophagi contained at least one focus of intraepithelial neoplasia, for the most part adjacent to the invasive carcinoma; in 14 per cent of the cases, intraepithelial carcinomas were detected at some distance from the invasive carcinomas. The invasive carcinomas involved an average of 20 per cent of the mucosal surface and the intraepithelial neoplasias, 5 per cent. The noncancerous mucosal lesions were analyzed, and their extent was evaluated. Esophagitis, parakeratosis, atrophy, dyskeratosis of the epithelium, and hyperplasia and metaplasia of the mucosal gland ducts were encountered. The exact significance of these lesions in esophageal carcinogenesis is not known. The mucosal abnormalities associated with preoperative irradiation are defined. The results of iodine and toluidine blue testing are presented.