We have analyzed the roles of tumor necrosis factor alpha (TNF-alpha) in human systemic lupus erythematosus (SLE) and murine models of lupus as well as in type 1 diabetes in NOD mice. These studies suggest an important role for TNF-alpha in the pathogenesis of autoimmune disease. Rather than being involved mainly in the effector arm of the inflammatory process of autoimmune organ destruction, our data suggest a primary involvement in some of the basic mechanisms of the autoimmune process. Evidence has been presented that emphasizes the possibility of the involvement of this cytokine in the genetic predisposition to SLE. The data may imply that the effect of TNF on the immune system may be more relevant to the pathogenesis of the autoimmune disease than direct local effects at some target organs. Based on the data presented, one should be cautious in extrapolating the effects of this cytokine in various in vitro systems to the in vivo situation.