In infections, microbial components provide signals that alert the immune system to danger and promote the generation of immunity. In the absence of such signals, there is often no immune response or tolerance may develop. This has led to the concept that the immune system responds only to antigens perceived to be associated with a dangerous situation such as infection. Danger signals are thought to act by stimulating dendritic cells to mature so that they can present foreign antigens and stimulate T lymphocytes. Dying mammalian cells have also been found to release danger signals of unknown identity. Here we show that uric acid is a principal endogenous danger signal released from injured cells. Uric acid stimulates dendritic cell maturation and, when co-injected with antigen in vivo, significantly enhances the generation of responses from CD8+ T cells. Eliminating uric acid in vivo inhibits the immune response to antigens associated with injured cells, but not to antigens presented by activated dendritic cells. Our findings provide a molecular link between cell injury and immunity and have important implications for vaccines, autoimmunity and inflammation.