BACKGROUND: Biliary cannulation failure is a major problem during endoscopic retrograde cholangiopancreatography. It remains unclear how duodenal papilla morphology affects biliary cannulation. Therefore, we proposed a new classification system for the duodenal papilla based on oral protrusion pattern (ratio of the length of the oral protrusion to the transverse diameter of the papilla) and papilla pattern. AIMS: To retrospectively compare biliary cannulation results with regard to classification and operator experience. METHODS: We analyzed 589 naïve major duodenal papillae. Our classification system comprised oral protrusion pattern, classified as small (Protrusion-S), regular (Protrusion-R), or large (Protrusion-L), and the papilla pattern, classified as annular (Papilla-A), unstructured (Papilla-U), longitudinal (Papilla-LO), isolated (Papilla-I), or gyrus (Papilla-G). Intra-evaluator concordance and the results of biliary cannulation were analyzed. RESULTS: The following oral protrusion pattern classifications were observed: Protrusion-S, 11.7%; Protrusion-R, 77.9%; and Protrusion-L, 10.4%. The following papilla patterns were observed: Papilla-A, 67.1%; Papilla-U, 7.0%; Papilla-LO, 7.5%; Papilla-I, 1.2%; Papilla-G, 15.6%; and unclassified, 1.7%. Intra-evaluator concordance value (Fleiss kappa) was 0.788 for oral protrusion pattern and 0.750 for papilla pattern. A logistic regression analysis of cannulations performed by an experienced endoscopist identified Protrusion-L as a significant risk factor for difficult cannulation (odds ratio 2.956; 95% confidence interval 1.115-7.84; p = 0.029). Multivariate analysis confirmed Protrusion-L as an independent risk factor for difficult biliary cannulation (odds ratio 3.772; 95% confidence interval 1.359-10.464; p = 0.011). CONCLUSIONS: We propose a new general classification system for the duodenal papilla. Protrusion-L is a significant risk factor for difficult biliary duct cannulation.