Although most patients with elevated body temperature have fever, there are circumstances in which elevated temperature represents not fever but hyperthermia (heat stroke). Hyperthermia is characterized by an uncontrolled increase in body temperature that exceeds the body’s abil- ity to lose heat. The setting of the hypothalamic thermoregulatory center is unchanged. In contrast to fever in infections, hyperthermia does not involve pyrogenic molecules. Exogenous heat exposure and endoge- nous heat production are two mechanisms by which hyperthermia can result in dangerously high internal temperatures. Excessive heat pro- duction can easily cause hyperthermia despite physiologic and behav- ioral control of body temperature. For example, work or exercise in hot environments can produce heat faster than peripheral mechanisms can lose i