Normal body temperature values are distributed in a Gaussian manner and are subject to circadian variation. Therefore, the usually accepted upper limit of 37 degrees C for normal body temperature should be replaced by a value of 37.1 degrees C in the morning and 37.4 degrees C in the afternoon. Fever develops when cytokines increase the thermostatic set point in the hypothalamus, which in turn results in increased body temperature via increased heat production and decreased heat dissipation. Hyperthermia is a distinct entity in which the thermostatic set point is normal but the heat control mechanism fails. Increased body temperature has positive effects (e.g. decreased bacterial growth, stimulation of host defence mechanisms) as well as negative effects (e.g. increased heart rate, oxygen consumption and metabolism). Whether fever is a friend or foe depends on the actual clinical circumstances. Antipyretic treatment should, therefore, not be applied routinely. In the case of pure hyperthermia (e.g. heat stroke), physical cooling is appropriate, while in the case of fever the thermostatic set point must first be normalized with drugs before cooling can be applied.