There have been uncertainties as to the role of common bile duct (CBD) stones in severe gallstone pancreatitis. In order to resolve this, ERCP findings in 131 patients with acute pancreatitis were compared with predicted severity, clinical course and final outcome. Significant associations were found between 'persisting' CBD stones, coincidental acute cholangitis, predicted severity and actual outcome. There was evidence for acute obstruction of both the CBD and the pancreatic duct by CBD stones. The theory was therefore proposed that small migrating stones tend to initiate the attack, whereas larger 'persisting' stones tend to convert a mild attack into a severe attack. This hypothesis resolves previously irreconcilable theories and lends support to the use of urgent endoscopic sphincterotomy for treatment, but only in cases predicted to be severe.