The role of routine isoamylase determinations in differentiating acute pancreatitis from other causes of an acute abdomen with hyperamylasemia and/or hyperamylasuria was evaluated. Values were analyzed from a control group of 21 patients with acute pancreatitis (group I) and from 100 consecutive patients diagnosed in our emergency department as having an acute abdomen (group II). In group I, 100% of patients had hyperamylasemia, hyperamylasuria, and a P isoamylase fraction greater than 0.75 of the total amylase value. In group II, 50% of patients had hyperamylasemia and/or hyperamylasuria. Of these patients, 44% had a P isoamylase fraction less than 0.75 of the total amylase value, a finding apparently incompatible with a diagnosis of acute pancreatitis as identified by our control group. We conclude that routine isoamylase determinations in patients with an acute abdomen and hyperamylasemia and/or hyperamylasuria allows the differentiation from acute pancreatitis in 44% of cases.