Between January 1973 and January 1989, 241 patients with Barrett's esophagus were treated at the Lahey Clinic Medical Center, Burlington, Mass. Of these patients, 65 presented with adenocarcinoma in Barrett's esophagus for a prevalence rate of 27%. Of 176 patients followed up for a total of 497 patient-years, adenocarcinoma developed in five patients for an incidence of one per 99 patient-years. The development of adenocarcinoma during endoscopic surveillance 1, 2, 2, 4, and 10 years after the initial diagnosis of Barrett's esophagus emphasizes the importance of long-term endoscopic and histologic surveillance. All five patients had severe dysplasia before adenocarcinoma developed. Yearly endoscopic follow-up examination is recommended for all patients with Barrett's esophagus unless mild dysplastic changes are found, in which case surveillance should be increased. Patients with severe dysplasia who are otherwise acceptable candidates for operation should be advised to have esophageal resection.