To analyze the relationship between Lugol unstained areas and their histologic features, we applied the Lugol test to 24 specimens of resected esophagus. The staining patterns were graded into four types: grade I, hyperstaining; grade II, normal greenish brown staining; grade III, less intense staining; and grade IV, unstained. Most of the grade IV lesions were invasive carcinomas, carcinomas in situ, or severe dysplasia. The carcinomas in situ and the intraepithelial extension of the carcinomas, which were difficult to detect, were clearly shown as grade IV. On the other hand, moderate to mild dysplasia or atrophy showed grade III staining. Grade IV lesions showed well-demarcated sharp margins, whereas grade III lesions showed ill-demarcated dull margins. The grade III carcinomas, however, by the Lugol test, showed well-demarcated margins. Histologic evaluation disclosed that the staining intensity reflected well the thickness of the glycogen-containing cell layer in the lesion. The sharpness of the margin reflected the abrupt or gradual change from the glycogen-containing to non-containing cell layers. These findings suggest 1) the usefulness of the staining pattern of the Lugol test for the diagnosis of esophageal lesions such as squamous cell carcinoma and severe dysplasia, and 2) the usefulness of the Lugol test for precise delineation of the proximal resection line during surgery of esophageal carcinomas with unexpected wide extension.