Although several tools are available for the detection of cerebral vasospasm after subarachnoid haemorrhage, it has remained difficult to identify vasospasm timely and accurately. INVOS® monitoring measures the oxygen saturation by using near infrared spectroscopy, and in this study we examined the usefulness of this system for the detection of vasospasm. Five patients who had suffered SAH were enrolled in this study. In view of the thickness of the clots, the probes of INVOS® were attached to the scalp in the areas where vasospasm was likely to occur, from day 3 to the end of vasospasm, up to 14 days post SAH. Patients were monitored every day by INVOS® and by neurological exams. Angiography was performed if regional oxygen saturation had continuously decreased by 10% or more as compared to the contralateral side, or if patients showed additional neurological deficits. If vasospasm was detected, interventional treatment using intraarterial fasudil injection or percutaneous transluminal angioplasty was performed. The same interventional therapy was repeated until neurological deficits improved. When the INVOS® value decreased to lower than 60, angiographic or symptomatic vasospasm tended to occur, and the response of regional oxygen saturation correlated accurately, both symptomatically and angiographically, with brain ischemia due to vasospasm. Recovery from ischemia on angiography or single photon emission CT also correlated with the return of regional oxygen saturation by INVOS®. INVOS® monitoring is handy and noninvasive; it is able to evaluate real-time regional oxygen saturation in the region of VS and may be superior to the other existing monitoring systems.