Leflunomide is a novel immunosuppressive and anti-inflammatory agent for the treatment of autoimmune disease. The aim of this study was to investigate whether leflunomide protects from liver injury induced by concanavalin A (Con A), a T-cell–dependent model of liver damage. BALB/c mice were injected with 25 mg/kg Con A in the presence or absence of 30 mg/kg leflunomide. Liver injury was assessed biochemically and histologically. Levels of circulating cytokines and expressions of cytokine messenger RNA (mRNA) in the liver and the spleen were determined. Treatment with leflunomide markedly reduced serum transaminase activities and the numbers of dead liver cells. Leflunomide significantly inhibited increases in plasma tumor necrosis factor alpha (TNF-α) and interleukin 2 concentrations, and also reduced TNF-α mRNA expression in the liver after administration of Con A. These findings were supported by the results in which leflunomide administration decreased the number of T lymphocytes infiltrating the liver as well as inhibiting their production of TNF-α. Activation of nuclear factor κB (NF-κB), which regulates TNF-α production, was inhibited in the liver of mice treated with leflunomide, resulting in a reduction of TNF-α production from lymphocytes infiltrating the liver. In conclusion, leflunomide is capable of regulating T-cell–mediated liver injury in vivo and that this event may depend on the decrease of TNF-α production in the liver through inhibition of NF-κB activation caused by leflunomide. (HEPATOLOGY 2004.)