Most attacks of acute pancreatitis are mild and self-limiting. In 10-20% of the cases, however, severe disease with multiple systemic complications develops. During the last few years it has been recognized that activated leukocytes have an important role in the multisystem involvement during acute pancreatitis. Activated leukocytes are thus a pathogenetic factor in the severity of the disease. Activation of polymorphonuclear granulocytes (PMNs) and of monocytes/macrophages is an early event during severe acute pancreatitis. Factors released by activated leukocytes therefore reflect the severity of the disease. Three independent studies have shown that released PMN-elastase is a reliable early prognostic marker that permits correct classification of 80-95% of the patients within the first 24-48 hours. Interleukin-6 (IL-6), mainly secreted by activated monocytes/macrophages, is also an early prognostic parameter (shown in one study), but is not superior to PMN-elastase. Leukocyte activation markers are more reliable than multiple scoring systems in the assessment of the severity of acute pancreatitis. Compared with PMN-elastase or IL-6, increased plasma concentrations of such acute-phase proteins as alpha-1-antitrypsin or CRP, and consumption of the protease inhibitor alpha-2-macroglobulin, are later events that can be detected only 1 to 4 days later. Comparison of the various inflammatory parameters suggests that PMN-elastase is the best early and reliable prognostic marker in acute pancreatitis. The reviewed data underscore the role of activated leukocytes in the pathogenesis of complicated acute pancreatitis.