AIM:To clarify the relationship between the change of serum amylase level and post-ERCP pancreatitis. METHODS:Between January 1999 and December 2002, 1291 ERCP-related procedures were performed. Serum amylase concentrations were measured before the procedure and 3, 6, and 24 h afterward. The frequency and severity of post-ERCP pancreatitis and the relationship between these phenomena and the change in amylase level were estimated. RESULTS:Post-ERCP pancreatitis occurred in 47 patients (3.6%). Pancreatitis occurred in 1% of patients with normal amylase levels 3 h after ERCP, and in 1%, 5%, 20%, 31% and 39% of patients with amylase levels elevated 1-2 times, 2-3 times, 3-5 times, 5-10 times and over 10 times the upper normal limit at 3 h after ERCP, respectively (level < 2 times vs >or= 2 times, P < 0.001). Of the 143 patients with levels higher than the normal limit at 3 h after ERCP followed by elevation at 6 h, pancreatitis occurred in 26%. In contrast, pancreatitis occurred in 9% of 45 patients with a level higher than two times the normal limit at 3 h after ERCP followed by a decrease at 6 h (26% vs 9%, P < 0.05). CONCLUSION:Post-ERCP pancreatitis is frequently associated with an increase in serum amylase level greater than twice the normal limit at 3 h after ERCP with an elevation at 6 h. A decrease in amylase level at 6 h after ERCP suggests the unlikelihood of development of post-ERCP pancreatitis.