The results presented here concern the study of in situ cancer and marked dysplasia revealed during the pathological study of 39 specimens removed during esophagogastrectomy for invasive carcinoma of the esophagus. In 12 cases, macroscopic study made it possible to define precisely the macroscopic features of in situ canccer; in one case, however, the mucous membrane at the site of the in situ cancer was macroscopically normal. The iodine test performed in 37 cases showed that the normal esophageal mucosa is iodine-positive and that in situ and invasive squamous cell carcinoma are always represented by sharply defined iodine-negative zones: in the case in which it was sufficiently extensive, marked dysplasia presented the same iodine-negative character. The possibilities for applying these results to early endoscopic diagnosis of esophageal cancer are presented.