Experimentally, focal and global cerebral ischemia are markedly affected by small changes in brain temperature. Mild hypothermia greatly ameliorates and mild hyperthermia markedly exacerbates ischemic-induced neuronal injury. Mild hypothermia not only protects against neuronal injury but also improves clinical outcome. This effect depends on the temperature of the brain both during and after the ischemic episode. Clinical and laboratory evidence concerning the value of hypothermia and the danger of hyperthermia in acute ischemia are presented. Potential mechanisms of action of hypo- and hyperthermia in ischemic injury are also addressed. The need to treat fever in acute cerebral ischemia, even if evidence is only empiric, and the intriguing possibility of using mild hypothermia to treat acute cerebral infarction are discussed.