AIMS: Barrett's oesophagus with indefinite for dysplasia (BE-IND) is a subjective diagnosis with a low interobserver agreement (IOA) among pathologists and uncertain clinical implications. This study aimed to assess the utility of p53 immunohistochemistry (p53-IHC) in assessing BE-IND specimens. METHODS AND RESULTS: Archive endoscopic biopsies with a BE-IND diagnosis from two academic centres were analysed. First, haematoxylin and eosin-stained slides (H&E) were reviewed by four expert gastrointestinal (GI) pathologists allocated into two groups (A and B). After a washout period of at least 8 weeks, H&E slides were reassessed side-to-side with p53-IHC available. We compared the rate of changed diagnosis and the IOA for all BE grades before and after p53-IHC. We included 216 BE-IND specimens from 185 patients, 44.0 and 32.9% of which were confirmed after H&E slide revision by groups A and B, respectively. More than half the cases were reclassified to a non-dysplastic BE (NDBE), while 5.6% of cases in group A and 7.4% in group B were reclassified to definite dysplasia. The IOA for NDBE, BE-IND, low-grade dysplasia (LGD) and high-grade dysplasia (HGD)/intramucosal cancer (IMC) was 0.31, 0.21, -0.03 and -0.02, respectively. Use of p53-IHC led to a >40% reduction in BE-IND diagnoses (P < 0.001) and increased IOA for all BE grades [κ = 0.46 (NDBE), 0.26 (BE-IND), 0.49 (LGD), 0.35 (HGD/IMC)]. An aberrant p53-IHC pattern significantly increased the likelihood of reclassifying BE-IND to definite dysplasia (odds ratio = 44.3, 95% confidence interval = 18.8-113.0). CONCLUSION: P53-IHC reduces the rate of BE-IND diagnoses and improves the IOA among pathologists when reporting BE with equivocal epithelial changes.