AIMS Primary sclerosing cholangitis (PSC) is characterized histologically by portal inflammation, bile duct injury and regeneration and concentric periductal fibrosis. Although seen commonly in our experience, the significance of histological thickening of the bile duct basement membrane on periodic acid Schiff (PAS)-positive, diastase-resistant (DPAS) staining has never been analysed formally. In this paper we provide an evidence-based assessment of basement membrane thickening (BMT) reproducibility and diagnostic accuracy. METHODS AND RESULTS A total of 128 archived medical liver core biopsies were retrieved and blinded for review by two independent histopathologists. BMT was assessed and designated as absent or present with a grade (G) of G1-G3. The sensitivity of any BMT for PSC was good at 77%, with moderate specificity at 61%. When only G3 BMT was considered positive, the specificity was high at 95% but the sensitivity was poor at 16%. The interobserver agreement (0.69) and consistency (0.72) were good. CONCLUSIONS Basement membrane thickening is a reproducible predictor for PSC with good sensitivity and specificity. The presence of G2 and especially G3 BMT showed high specificity and could be regarded as highly predictive of PSC. The presence of more than G1 BMT should be reported and the possibility of PSC should be raised in the differential diagnosis.