Pancreatitis is a common complication of endoscopic sphincterotomy. Cautery-induced papillary edema has been implicated as a possible cause. The objective of this study was to determine whether prophylactic stenting of the main pancreatic duct after endoscopic sphincterotomy in high-risk patients would reduce the incidence of pancreatitis. High-risk patients were defined as those with sphincter of Oddi dysfunction, small common bile duct diameter (< 10 mm), or those requiring pre-cut sphincterotomy. Patients were studied in a prospective fashion from October 1990 to April 1992 and were randomized to receive either a main pancreatic duct stent or no stent after biliary sphincterotomy. The stents were generally removed 10 to 14 days after placement. Fifty patients were randomized to the no-stent group and 48 patients were randomized to the stent group, but in five patients stent placement was unsuccessful. Pancreatitis occurred in 18% of patients in the no-stent group compared with 14% of patients in the stent group. Most cases of pancreatitis were mild, occurring in 10% of patients in the no-stent group and 12% of patients in the stent group. Moderate to severe pancreatitis occurred with an increased frequency in patients in the no-stent group (8%) compared with that in patients in the stent group (2%). Mean number of hospital days required to treat pancreatitis was 9.5 days in the no-stent group compared with 2.8 days in the stent group; however, none of these differences reached statistical significance. Small common bile duct diameter (< 6 mm) was found to be an independent risk factor for pancreatitis after endoscopic sphincterotomy.(ABSTRACT TRUNCATED AT 250 WORDS)