BACKGROUND/AIMS: The generation of oxygen-derived free radicals has been implicated in the pathogenesis of experimental pancreatitis. The aim of this study was to determine the role of oxygen-derived free radicals in patients with acute pancreatitis. METHODS: The serum levels of CRP, lipid peroxide (LPO), myeloperoxidase (MPO) and superoxide dismutase (SOD) were measured in 13 patients with acute pancreatitis and 14 healthy volunteers. RESULTS: The serum levels of LPO and MPO were higher and the activity of SOD was lower in patients with acute pancreatitis than controls. The serum levels of LPO and MPO showed higher tendency in the patients with severe pancreatitis than in patients with mild pancreatitis. However, there was no significant difference in the serum marker of oxidative stess according to the etiology. The LPO level was especially correlated with the concentration of serum CRP and CT severity index. CONCLUSIONS: The oxygen-derived free radicals may be closely associated with inflammatory process and the severity of acute pancreatitis. Especially, the concentration of serum LPO is meaningful as an index for severity of the disease.