Intestinalized epithelium in traditional long-segment Barrett's esophagus (BE) shows increased proliferative activity, which is postulated to be an early step in the metaplasia-dysplasia-carcinoma sequence. The aim of this study was to evaluate the proliferative activity of intestinalized epithelium of the distal esophagus and gastroesophageal junction (IMEGEJ). Tissue sections from 78 consecutive patients (20 with IMEGEJ, 58 without IMEGEJ) who had elective upper gastrointestinal endoscopy over a 6-month period were immunohistochemically stained with MIB-1, the Ki-67 proliferation-antigen-associated marker, for evaluation of the crypt MIB-1 proliferation index (PI), size of the proliferative zone (PZ), and the presence of surface epithelial staining. Data from the IMEGEJ and non-IMEGEJ groups, and from 15 age-matched patients with traditional long-segment BE (>3.0 cm), were compared statistically. IMEGEJ patients showed a statistically significant increase in the mean crypt PI compared with non-IMEGEJ controls (21.9+/-19.5 v 14.3+/-9.3; P=.01). In addition, IMEGEJ cases showed an increase in the mean crypt PZ (52.3+/-16.4 v 45.2+/-17.2; P=.05), and a trend toward an increase in the percentage of cases with MIB-1-positive surface epithelial cells (50% v 33%, P=.18). Patients with IMEGEJ did not differ from patients without IMEGEJ with respect to any other clinical or histological feature, including signs or symptoms of gastroesophageal reflux disease and presence or absence of esophagitis or carditis. The MIB-1 results of the patients with long-segment BE (MIB-1 PI = 22.6+/-20.5, MIB-1 PZ = 51.8+/-19.6, proportion of cases with MIB-1-positive surface cells = 66%) were similar to those with IMEGEJ. Intestinalized epithelium in the distal esophagus or gastroesophageal junction shows increased proliferative activity in comparison with patients without intestinalized epithelium. This finding supports an increased risk of carcinogenesis in patients with IMEGEJ.