Aspiration and dissection of a cystic tumor from the third ventricle of a 3-year-old child resulted in several hypothalamic symptoms. One of the most persistent and troubling symptoms was marked hyperthermia which could not be traced to an infectious origin. The elevated temperature was not reduced by acetaminophen and there were abnormalities of circadian temperature cycles. In thermal stimulation tests the high body temperature was defended against induced cooling but the body temperature was raised above 40 degrees C without evoking physiological heat-loss responses and thermal discomfort. The unusual thermoregulatory status of this patient is similar to that seen in laboratory animals with hypothalamic lesions and to that observed with certain naturally occurring hypothalamic lesions in man. This dysthermia has been successfully treated with chlorpromazine.