Nuclear factor-kappaB (NF-kappaB) is a potent mediator in several steps of acute pancreatitis. Leflunomide is a novel immunomodulating drug that is also a potent inhibitor of NF-kappaB activation. The aim of this study was to investigate the effects of leflunomide pretreatment in severe necrotizing pancreatitis in rats. Fifty rats were randomly divided into 5 groups. Severe necrotizing pancreatitis was induced by retrograde injection of 3% sodium taurocholate into the common biliopancreatic duct. Leflunomide (10 mg/kg) was given intragastrically for 2 doses before the experiment. Serum amylase activity, pancreatic histopathologic condition, malondialdehyde level, myeloperoxidase enzyme activity, nitric oxide level, and pulmonary changes were assessed. Leflunomide pretreatment significantly ameliorated pancreatic hemorrhage, edema, and neutrophil infiltration and decreased histopathological score compared with the untreated severe necrotizing pancreatitis group (pathological score [mean +/- SEM]: 6.70 +/- 1.19 vs 12.36 +/- 1.08 in the leflunomide treated and untreated groups, respectively, P < 0.01). Pulmonary changes was decreased in the leflunomide treated group (3.90 +/- 0.45 vs 4.75 +/- 0.25, respectively). Change in pulmonary alveolar distention was significant. Although serum amylase levels also decreased, the difference was not significant (5922 +/- 3290 vs 15547 +/- 5090 U/mL). Leflunomide is a beneficial agent in the severe form of acute pancreatitis in rats and should be considered as a potential agent for treatment of acute pancreatitis.