Since 1983 a clinical trial of proton beam radiotherapy has been conducted at the Proton Medical Research Center (PMRC) of the University of Tsukuba. We have made it a rule to do field localization by X-ray pictures before each treatment. For this purpose we have developed a localize-verify system consisting of a fluoroscopic unit and a real time digital image processing device. By using this system as well as X-ray films, field placement errors or corrected distance at field localization were measured in 11 patients with esophageal cancers. Measurements of corrected distances on a total of 177 localization attempts disclosed that correction by > 5 mm was necessary in 30.6% and by > 10 mm in 10.2% of all localization attempts. Corrected distances appeared to increase with age, possibly because the skin becomes looser and ambulatory status tends to be more limited in older patients. Field placement corrections of more than 5 mm were required in 66.7% of 60 localizations in patients > 80 years old. Two patients in whom the anatomical positions of the esophagus were easily movable are presented. The following common characteristics of these patients were considered high risk factors: they were more than 80 years old; lesions were located in the lower esophagus; and they had T1 tumors. These findings suggested that frequent positioning and verification of treatment fields are necessary in the accurate treatment of esophageal cancers, especially those in high-risk patients.