Although it has been known for a long time that pronounced hypothermia has a protective effect on the brain during ischemia, and that severe hyperthermia damages neuronal tissue, knowledge of human brain temperature is very limited. The recent findings by two independent research groups, that even small differences in brain temperature significantly influence the degree of neuronal damage following cerebral ischemia, became the incentive for measuring brain temperature in neurosurgical patients. The temperature of the lateral ventricle, epidural space, membrana tympani and rectum were measured with copper-constantan thermocouples. During the implantation of an intraventricular catheter for measuring intracranial pressure, a temperature gradient of 0.4-1.0 degrees C between the lateral ventricle and the epidural space was noted. Continuous measurements for 1-5 days showed that the rectal temperature usually adequately reflects the temperature of the epidural space, although the temperature of the membrana tympani followed changes in epidural temperature more closely. However, at times, and in one patient during most of the time, the temperature of the epidural space was up to 1 degree C above rectal temperature. The relevance of these findings for the care of neurosurgical patients is discussed in relationship to what is known about brain temperature from animal experiments.