The levels of amylase, trypsinogen, and trypsin-alpha 1-protease inhibitor complexes, both in serum and in peritoneal fluid, were correlated to the severity and clinical course in 27 attacks of acute pancreatitis. Serum levels of trypsin-alpha 1-protease inhibitor complexes on admission correlated well with the severity and clinical course of the disease, whereas serum levels of amylase and trypsinogen did not. This may be of clinical importance in differentiating severe acute pancreatitis from milder and self-limiting forms of the disease.