We evaluated the risk of adenocarcinoma developing in Barrett's esophagus (esophagus lined with columnar epithelium). Mayo Clinic records were reviewed, and all cases that met predefined histologic criteria for the diagnosis of Barrett's esophagus in 1979 or earlier were included. In 18 of 122 such cases, adenocarcinoma of the esophagus and Barrett's esophagus were diagnosed simultaneously. The status of the remaining 104 cases was determined after a mean interval of 8.5 years. During this time, adenocarcinoma of the esophagus developed in 2 patients, and 24 died from other causes. We conclude that although the incidence of esophageal adenocarcinoma is increased in patients with symptomatic Barrett's esophagus, it does not occur in the majority of such patients.