Serum amylase shows the greatest increase among the various pancreatic enzymes that increase at the onset of acute pancreatitis. However, the diagnostic value of the total serum amylase activity has been questioned due to its lack of specificity. To differentiate hyperamylasemia due to pancreatic disease from that due to other causes, the activity of pancreatic amylase should be determined by using a monoclonal antibody that specifically binds to pancreatic or salivary amylase, or by electrophoresis. The most useful and accurate method for distinguishing pancreatic from salivary-type hyperamylasemia is isoamylase analysis by electrophoresis. In patients with acute pancreatitis, increase of Amylase-1 and -2 is accompanied by the appearance of Amylase-4, a minor component of the pancreatic-type isoamylases, and by disappearance of the salivary-type isoenzymes, thereby leaving a pattern of the pancreatic isoenzymes alone. This pancreatitis pattern persists for about 10 days after the onset of illness. Therefore, if such a pattern is found in a patient with clinical findings suggesting acute pancreatitis despite a normal serum amylase level, the patient can be diagnosed as having acute pancreatitis or a recent attack of the disease. However, the existence of an inherited trait of the pancreatitis pattern in some healthy individuals must be borne in mind. Patients with recurrent chronic pancreatitis also show pancreatic-type hyperamylasemia, whereas the pancreatic amylase activity decreases when pancreatic exocrine insufficiency progresses. Hyperamylasemia due to elevated salivary amylase activity is also common in patients with diabetic ketosis or malignancies such as lung cancer (adenocarcinoma). Hyperamylasemia is also found following various types of operation. In most cases, it is salivary-type hyperamylasemia.(ABSTRACT TRUNCATED AT 250 WORDS)