Levels of leukocyte elastase and neutrophil protease 4 (NP4(3)) in plasma and peritoneal exudate were studied in 25 patients with severe, acute pancreatitis. Pancreatitis was diagnosed from the clinical picture and an increased serum amylase level. The diagnosis was verified by computerized tomography, ultrasound, and findings at operation or autopsy. Peritoneal exudate on admission contained high concentrations of leukocyte elastase (6100 +/- 2000 micrograms/l) and NP4(3) (2310 +/- 900 micrograms/l). High initial levels were found also in plasma, which contained 659 +/- 110 micrograms/l of leukocyte elastase and 254 +/- 33 micrograms/l of NP4(3). The levels in plasma were still increased 3 weeks after the acute attack, also in the absence of complications, indicating that the resolution of acute pancreatitis is a protracted process. Plasma levels of both leukocyte proteases were persistently increased in patients with pancreatic abscess, in contrast to the gradual decrease seen in patients with a pseudocyst or uncomplicated recovery. The levels were increased already before the abscess was diagnosed clinically, which indicates that determinations of leukocyte elastase and NP4(3) may be helpful in detecting this complication. A pathophysiologic role for leukocyte proteases in the development of severe, acute pancreatitis should be considered.