BACKGROUND Vasospasm is a frequent complication of subarachnoid hemorrhage (SAH), implicated in poor outcome, and prediction of its occurrence might be important on the therapeutic approach of this condition. Brain oxygenation in aneurysmal SAH was prospectively monitored during surgery in 28 patients, using brain tissue oxygen pressure (PbtO(2)), studying its association with the occurrence of post-operative vasospasm, detected by transcranial Doppler. METHODS PbtO(2) monitoring was performed during surgery of 28 patients with aneurysmal SAH, using a polarographic microcatheter (Licox; GMS, Kiel, Germany), inserted into the cerebral tissue. The aneurysms were localized in the anterior communicating artery (AcoA) complex (eight cases), in the posterior communicating artery (Pcom) origin (seven cases), and in the middle cerebral artery (MCA) bifurcation (13 cases). Basal PbtO(2) values, obtained immediately before application of temporary or definitive clips, were studied according to age, clinical status and CT findings. The association between the basal values and the occurrence of increased blood flow velocity in the post-operative transcranial Doppler (TCD vasospasm) was investigated. RESULTS Post-operative TCD vasospasm developed in 13 patients, all of them with basal values inferior to 10 mmHg. PbtO(2) basal value was significantly lower in cases that developed TCD vasospasm. This association was independent of age, clinical status, or CT findings. CONCLUSIONS The finding of low intraoperative basal PbtO(2) values may be an indicator for a high risk of occurrence of post-operative TCD vasospasm in cases of aneurysmatic SAH.