BACKGROUND: Recent experimental and clinical evidence of hypothermic protection against neuronal injury creates new interests regarding human brain temperature. However, very little information is available for the brain temperature under certain pathological conditions. In this study, intra-operative brain temperature in patients with subarachnoid haemorrhage (SAH) is particularly addressed. METHODS: Brain surface temperature and oxygen saturation of jugular bulb (SjO2) were monitored during early surgery undergone within 48 hours after the onset in patients with SAH (n = 16). Those were also measured in patients with unruptured aneurysms during elective surgery as control (n = 15). FINDINGS: The brain surface temperature was significantly lower in SAH than control (35.3 +/- 0.8 vs. 36.1 +/- 0.5 degrees C, P < 0.01). The reduction in brain surface temperature was correlated with the severity of the Hunt and Kosnik's aneurysmal grade (r = 0.837, P < 0.01). SjO2 was significantly lower in SAH than control (51.5 +/- 7.3 vs. 68.5 +/- 7.6%, P < 0.01), and was positively correlated with brain surface temperature (r = 0.642, P < 0.01). INTERPRETATION: These results suggest that the brain temperature and/or the temperature gradient within the brain may be altered in an early period after SAH. Since brain temperature is determined by cerebral blood flow (CBF), metabolism, temperature of both circulating blood and surrounding environment, the brain surface temperature reduction may be explained by depressed CBF and metabolism in SAH.