Hyperthermia in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. It is usually due to excessive exposure to heat. The heat-regulating mechanisms of the body eventually become overwhelmed and are unable to effectively deal with the heat, and body temperature climbs uncontrollably. This is a medical emergency that requires immediate medical attention.

Hyperthermia can be created artificially by drugs or medical devices. In these instances it may be used to treat cancer and other conditions. Malignant hyperthermia is a rare complication of some types of general anesthesia.

Hyperthermia is the opposite of hypothermia.