OBJECTIVE: A small common bile duct (CBD) diameter has been associated with complications and with a difficult biliary cannulation. Previous studies suggested that this diameter can be predicted during the endoscopic retrograde cholangiopancreatography (ERCP) simply by observing the papillary morphology. Despite this published suggestion there is no study addressing this topic. This study evaluated a possible association between the morphology of the major papilla and the diameter of the terminal CBD (t-CBD). METHODS: Observational cross-sectional study including consecutive patients with naïve papillae was referred for ERCP in two affiliated university hospitals. The transverse (p-transv) and longitudinal measures (p-long) of the papilla were obtained using a visual method. Papillae were classified into nonprominent, prominent, bulging or other. The t-CBD was measured 1 cm from the papilla using fluoroscopic images in prone/supine. Measurements were performed by two senior endoscopists and outcomes were evaluated using correlation and linear regression model. RESULTS: We included 245 patients with a median age of 76 years. The median p-transv for each type of papillae was as follows: nonprominent = 6 mm, prominent = 9 mm, bulging = 15 mm and other = 6 mm; P < 0.001. The median t-CBD for nonprominent = 7.62 mm, prominent = 8.34 mm, bulging = 8.60 mm and other = 8.52 mm; P = 0.40. The correlation between the transverse and longitudinal measures of papilla and the t-CBD were 0.0092 and 0.0614, respectively. In the regression model, the t-CBD diameter was not explained by papilla's size or morphology (R = 1.70%; P = 0.80). CONCLUSION: The morphology of the papilla must not be used as a predictor of the diameter of the CBD as there is no correlation between these two items.