In a retrospective histological study, resected specimens obtained from 23 patients with adenocarcinoma in Barrett's esophagus (Group I) and endoscopic multiple (step) biopsies from 38 patients without carcinoma in Barrett's esophagus (Group II) were investigated for dysplastic changes. Dysplasia was most frequently found in the type of mucosa comprising intestinal metaplasia. There seem to be two pathways to dysplasia in Barrett's esophagus. In Group I dysplasia was found in 18 out of 23, and in Group II in 2 out of 38 patients. In Barrett's esophagus, dysplasia may be considered not only a precursor of carcinoma, but also a marker for coexisting carcinoma.