This report describes two patients with esophageal columnar epithelium (Barrett's esophagus) in which microinvasive adenocarcinoma developed. Case 1 had multiple foci of carcinoma in situ contiguous with epithelium resembling gastric and intestinal mucosa. Case 2 had signet-ring type adenocarcinoma. Surveillance for malignant transformation in columnar esophageal epithelium should be routinely performed, and because of its focal nature, multiple biopsies and cytologic examination should be carried out. The presence of carcinoma in situ should lead to consideration of excision of the affected esophagus.