Between 1978 and 1989, 13 of 48 patients with anomalous union of the pancreaticobiliary ductal system (AUPBD) were diagnosed as having acute pancreatitis. We have studied the clinical, radiologic, and surgical features of these 13 patients. A transient rise in the intraductal pressure of the pancreatic duct during an episode of abdominal pain is responsible for pancreatitis in patients with AUPBD. This rise in the intraductal pressure must be due to bile reflux into the pancreatic duct when an abnormally long common channel is blocked by cholelithiasis, protein plug, or dysfunction of the sphincter of Oddi. The pancreatitis resolves when the common channel obstruction is removed, and bile and pancreatic juice flow easily into the duodenum. We believe that this phenomenon is responsible for acute relapsing pancreatitis. It is our belief that the pancreas appears almost normal during symptom-free intervals.