There is general acceptance that a diagnosis of acute pancreatitis requires two of the following three features: 1) epigastric or upper abdominal pain characteristic of acute pancreatitis, 2) serum amylase and/or lipase ≥3 times the upper limit of normal, and 3) characteristic findings of acute pancreatitis on abdominal images such as ultrasonography, computed tomography (CT) or magnetic resonance imaging. Other pancreatic diseases and acute abdomen have been ruled out before making a diagnosis of acute pancreatitis. Serum lipase may be more useful than serum amylase because serum lipase is thought to be more sensitive and specific than serum amylase in the diagnosis of acute pancreatitis. Contrast-enhanced CT scan is the best imaging technique to rule out the conditions that masquerade as acute pancreatitis, to diagnose the severity of acute pancreatitis, and to identify complications of pancreatitis. The etiology of acute pancreatitis should be made clear as soon as possible to decide treatment policy of acute pancreatitis or to prevent the recurrence of pancreatitis. The etiology of acute pancreatitis in an emergency situation should be assessed by clinical history, laboratory tests such as serum liver function tests, measurement of serum calcium and serum triglycerides and ultrasonography. A differentiation of gallstoneinduced acute pancreatitis should be given top priority in its etiologic diagnosis because early endoscopic retrograde cholangiopancreatography should be considered if a complication of cholangitis and a prolonged passage disorder of the biliary tract are suspected.