The main goal of this study was to evaluate the development of adenocarcinoma in patients with Barrett's esophagus. During the period from January 1975 to December 1985, a total of 134 patients had endoscopically severe esophagitis and/or Barrett's esophagus. In these patients, 32 (24%) met the macroscopic and histologic criteria for the diagnosis of Barrett's esophagus. A check-up study of these patients was performed in 1987. Adenocarcinoma developed in three patients during the follow-up period of 166.1 patient-years. Dysplasia in the columnar epithelium was found in two of these patients six and 15 months before the diagnosis of adenocarcinoma. The third patient with adenocarcinoma was detected in endoscopic follow-up in 1987. In addition, the endoscopic examination showed unchanged Barrett's epithelium in all but three patients despite the operative and/or medical treatment 3-12 years (mean 6.7 years) earlier. We conclude that Barrett's esophagus is a potential premalignant condition and careful endoscopic surveillance for dysplasia in the columnar epithelium of the distal esophagus is mandatory in patients with Barrett's esophagus.