Interleukin 10 (IL-10) recently emerged as an antiinflammatory cytokine that inhibits the secretion of proinflammatory cytokines by monocytes and/or macrophages and the release of free oxygen radicals. It has been reported that treatment with IL-10 decreases the severity of experimental pancreatitis, mainly by inhibiting cellular necrosis. The aim of this study was to evaluate the behavior of serum IL-10 in patients with acute pancreatitis and to explore the possibility of a relationship between this cytokine and severity of the disease. Forty-five patients with acute pancreatitis were studied. Acute pancreatitis was of biliary origin in 30 patients, due to alcohol abuse in 10, due to pancreas divisum in 1, and of unknown origin in the remaining 4. According to the Balthazar criteria, 19 patients had scores of A, B, or C and 25 had scores of D or E. Twelve healthy subjects were also studied as controls. Serum IL-10 was determined in all subjects on admission, and in acute pancreatitis patients also daily for the following four days using a commercial kit. Healthy subjects had no detectable serum levels of IL-10. In acute pancreatitis patients, serum IL-10 levels were increased on the first day of the disease and then progressively decrease in the following days. On the first day of the acute pancreatitis, patients with the mild disease had serum levels of IL-10 significantly higher than those with severe disease, whereas in the following days, no statistically significant difference was observed between the two groups. The elevation of IL-10 on the first day of the illness is more marked in patients with mild acute pancreatitis than in those with the severe form of the disease. The finding of low values of serum IL-10 in severe acute pancreatitis suggests that there may be altered down-regulation of the immune system response in these patients.