To assess changes in caliber of the main pancreatic duct, we performed abdominal ultrasonography after maximal stimulation with secretin (US-S test) in 14 patients with idiopathic recurrent acute pancreatitis, in six with recurrent acute pancreatitis secondary to pancreas divisum, in 14 recovered from a single attack of acute pancreatitis, and in 21 control subjects. In five patients, the test was repeated 10 days after endoscopic sphincterotomy. We repeated the test 48 h later in nine subjects to evaluate its reliability. We evaluated changes in lipase serum values in some of these subjects. In the acute pancreatitis patients, the main pancreatic duct diameter was significantly increased over baseline and control values throughout the observation period. In the patients undergoing sphincterotomy, the poststimulation diameter of the main duct was substantially reduced after the operation. The reliability of the test ranged from 77 to 91.5%. In the acute pancreatitis patients, serum enzymes after secretin stimulation showed a persistent increase over controls. These results suggest that pancreatic outlet obstruction, mainly at the sphincter of Oddi level, may be an important pathogenetic factor in the course of the disease and that, if this condition is present after an attack of acute pancreatitis, endoscopic sphincterotomy may be in order. The simplicity and satisfactory reproducibility of the US-S test suggest a strong case for its routine clinical use.