Background/Aims The protease inhibitors, nafamostat and gabexate, have been used to prevent pancreatitis related to endoscopic retrograde cholangiopancreatography (ERCP). In vitro, nafamostat inhibits the pancreatic protease activities 10-100 times more potently than gabexate. We evaluated the efficacy of nafamostat for prophylaxis against post-ERCP pancreatitis in comparison with gabexate. Methods Five hundred patients (208 patients in the nafamostat-treated group and 292 in the gabexate-treated group) were analyzed retrospectively after selective exclusion. The incidences of pancreatitis and hyperamylasemia after the ERCP were compared between the nafamostat and gabexate groups. Results The incidences of acute pancreatitis and hyperamylasemia were 9.1% and 40.9%, respectively, in the nafamostat-treated group, and 8.6% and 39.4% in the gabexate-treated group. The frequencies of post-ERCP pancreatitis and hyperamylasemia did not differ significantly between the two groups, Post-ERCP pancreatitis in two group did not vary according to the different ERCP procedures. The mean serum amylase level at 6 h after ERCP was significantly lower in the nafamostat-treated group than in the gabexate-treated group (p=0.020). However, the difference in serum amylase level did not persist at 18 h and 36 h post-ERCP. Conclusions Administration of nafamostat before ERCP was not inferior to gabexate in protecting against the development of pancreatitis.