Mild hypothermia may occur spontaneously or, because of its putative neuroprotective effect, may be induced purposefully during neurosurgical procedures. Though the brain is the organ targeted for the purpose of neuroprotection, little is known about its temperature during general anaesthesia and craniotomy. The purpose of this study was to define the relations between core, skin and brain temperature during craniotomy and to compare two modes of inducing thermal insulation in patients during operative procedures. To achieve this we recorded core: rectal (Tre), oesophageal (Tes) and tympanic (Try) temperature, brain temperature in the subdural space (Tsd), and skin temperature on the thigh (Tfe), forehead (Tfr) and sternum (Tst) in 15 patients undergoing standard procedure for aneurysm clipping. In 13 patients the core temperature decreased, whereas skin temperature increased, after induction of general anaesthesia with isofluran. Nevertheless the mean body temperature remained unchanged, thus supporting the view that the cause of the resultant core hypothermia was heat redistribution between the thermal core and the periphery. Special thermofoil proved to be only as effective as a plain cotton blanket in preventing further heat loss during the later phases of the operation. Brain temperature was found to be the lowest core temperature throughout the procedure. It differed by as much as 0.1-1.2 degrees C from rectal temperature (mean 0.75 +/- 0.41 degree C) and reached the level of mild hypothermia (below 35 degrees C) even in those patients in whom rectal temperature indicated the state of normothermia. Furthermore tympanic and oesophageal temperature was on average 0.5 degree C higher than brain temperature. In conclusion, temperature measurements obtained in standard sites do not reflect brain temperature reliably during craniotomy and general anaesthesia. This indicates that the direct measurement of intracranial temperature is necessary for correct estimation of brain hypothermia.