The biliary tract has been prospectively studied in a consecutive series of 769 patients undergoing surgery for gallstones to determine whether differences exist between subjects with and without a history of acute pancreatitis. The incidence of acute gallstone pancreatitis (AGP) was 7.7 per cent and men with gallstones were significantly more likely to develop pancreatic inflammation. Operations on patients with AGP were accompanied by a higher mortality rate which was almost entirely due to the severity of the disease at the time of surgery. The earlier operations were performed after the onset of pancreatitis the more often stones were found in the common bile duct and at the ampulla. Patients with AGP had smaller and more numerous gallbladder stones in association with a wider cystic duct that controls. The common bile duct diameter in patients with AGP was independent of the presence of choledochal calculi implying either previous temporary obstruction to the biliary tree or a dilated duct ab initio. Pancreatic duct reflux was far more commonly observed on the cholangiograms of patients with AGP and in these patients reflux occurred into a wider pancreatic duct, at a greater angle and was associated with a longer functioning common channel. No patient developed recurrent pancreatitis following biliary surgery. These features strongly support the concept of gallstone migration and suggest that patients with gallstones who develop acute pancreatitis have essential differences in their biliary tree which mechanically facilitate migration of calculi.