OBJECTIVE During intracranial aneurysm surgery, numerous factors may alter cerebral blood flow and oxygen supply-demand balance. Continuous monitoring of jugular bulb venous oxygen saturation (SvjO2) may help in the anesthetic management of such procedures. METHODS Fiberoptic SvjO2 was continuously monitored in seven patients during intracranial aneurysm surgery. Fiberoptic SvjO2 measurement was compared with IL3 CO-OXIMETER determination from 85 paired samples. The occurrence of large SvjO2 variations (SvjO2 variation reaching 10% or more of stable preceding value) during aneurysm surgery was recorded and classified according to the association or not with systemic clinical or therapeutic changes. RESULTS Fiberoptic SvjO2 showed a limited accuracy, with limits of agreement with IL3 CO-OXIMETER at -16.8% and +10.7% and a small bias (-3.1%). SvjO2 variations were frequent during aneurysm surgery, ranging from 3 to 22 per patient during procedures lasting 6 hours (range 4.5 to 7). Half of these variations occurred in the absence of any systemic clinical or therapeutic change, most often leading to an increased SvjO2. CONCLUSIONS Although the accuracy of fiberoptic SvjO2 determination is limited, it allows the detection of cerebral blood flow and oxygen supply-demand imbalance during aneurysm surgery. The frequent occurrence of SvjO2 elevations is suggestive of reactive hyperemia mechanisms.