An accurate history and thorough physical examination will often raise clinical suspicion of acute pancreatitis in the differential diagnosis of a patient presenting with acute abdominal pain. An accurate diagnosis is needed to eliminate etiologies of acute abdominal pain and to appropriately direct therapy. Confirmation of the diagnosis is most often made by evaluation of serum amylase and lipase levels. Although hyperamylasemia is found in the majority of patients with acute pancreatitis, other nonpancreatic acute abdominal conditions may be present with hyperamylasemia. CT scanning provides an accurate confirmation of clinical and laboratory findings and offers excellent anatomic and morphologic representation of the pancreas and peripancreatic tissue. The following article, written by the late John H.C. Ranson, presents a discussion of the modalities available for diagnosing acute pancreatitis.