The purpose of this study was to determine if routine isoamylase assay would provide valuable diagnostic information in patients with hyperamylasemia. Isoamylase distribution was determined in sera of 37 consecutive hyperamylasemic patients. The attending physicians (without knowledge of the isoamylase level) had considered acute pancreatitis to be “probable” in 19, “possible” in 4, and “unlikely” in 14 of these 37 patients. Three of the patients considered probably to have pancreatitis and 3 thought possibly to have pancreatitis had normal serum pancreatic isoamylase levels. Knowledge of the normal pancreatic isoamylase level in these 6 patients probably would have changed the clinical diagnosis to some condition other than pancreatitis. Of the 14 hyperamylasemic patients thought “unlikely” to have pancreatitis, 7 had an elevation of just pancreatic isoamylase and 3 additional patients had elevation of both pancreatic and salivary isoamyIases. It seems likely that knowledge of these elevated pancreatic isoamylase levels would have changed the clinical diagnosis to “probable” pancreatitis for many of these patients. We conclude that routine isoamylase assay provides diagnostic information that might change the clinical diagnosis in 20%–40% of hyperamylasemic patients.