BACKGROUND AND AIMS: When to perform oesophagectomy for neoplastic progression in Barrett's oesophagus is controversial. Some resect for high grade dysplasia whereas others defer treatment until intramucosal adenocarcinoma is diagnosed. Interobserver agreement for a diagnosis of high grade dysplasia or intramucosal adenocarcinoma remains unknown and may have therapeutic implications. METHODS: Histological slides from 75 oesophagectomy specimens with high grade dysplasia or T(1) adenocarcinoma were blindly reviewed by two gastrointestinal pathologists and one general surgical pathologist, and classified as high grade dysplasia, intramucosal adenocarcinoma, or submucosal adenocarcinoma. A subsequent re-review of all 75 cases by the same observers following establishment of uniform histological criteria was undertaken. Interobserver agreement was determined by kappa statistics. Coefficients <0.21, 0.21-0.40, 0.41-0.60, 0.61-0.80, and >0.80 were considered poor, fair, moderate, good, and very good agreement, respectively. RESULTS: Interobserver agreement among all pathologists and between gastrointestinal pathologists when comparing high grade dysplasia with intramucosal adenocarcinoma was only fair (k=0.42; 0.56, respectively) and did not substantially improve on subsequent re-evaluation following establishment of uniform histological criteria (K=0.50; 0.61, respectively). CONCLUSIONS: When evaluating resection specimens and after implementation of uniform histological criteria, even experienced gastrointestinal pathologists frequently disagree on a diagnosis of high grade dysplasia versus intramucosal adenocarcinoma. Treatment strategies based on the histological distinction of high grade dysplasia from intramucosal adenocarcinoma using limited biopsy specimens should be re-evaluated.