During the past several years, it has become increasingly apparent that interleukin-1 (IL-1), particularly IL-1 beta plays an important role in brain injury during ischemia. Studies from various laboratories have shown that IL-1 beta mRNA and IL-1 beta protein are synthesized early in ischemia and that the injection of IL-1 beta into ischemic brain enhances edema formation. The most direct evidence that IL-1 beta contributes to ischemic injury, however, is the demonstration that infarct volume in focal ischemia is reduced following intraventricular injection of an endogenous interleukin-1 receptor antagonist (IL-1ra), or after IL-1ra is overexpressed in brain using an adenoviral vector to transfer IL-1ra cDNA to brain cells. Ischemic injury is also reduced in mice that fail to produce IL-1 beta because of an abnormal interleukin-1 beta converting enzyme gene (ICE knockout mice). At the present time, it is nuclear how IL-1 beta causes brain injury, but several possible mechanisms include 1) stimulation of an inflammatory response through the activation of glia or the induction of other cytokines and/or endothelial adhesion molecules and 2) release of free radicals through stimulation of arachidonic acid metabolism and/or nitric oxide synthase activity.