We tested the hypothesis that transcranial oximetry by fast scanning near infrared spectroscopy can detect local desaturation of hemoglobin in arterial vessels of cerebral circulation with impaired blood supply. A total of 74 near infrared spectroscopy recordings were taken from the intact skull of humans. Perfusion of the hemisphere under the detector was assessed in one of four groups: (1) healthy volunteer; (2) patient, unaffected side; (3) patient, affected side with intact collateral blood supply; (4) patient, affected side, impaired collateral blood supply. Transcranial saturation was 0.90±0.01 (all values reported as mean±S.E.) in healthy volunteers (n=24), 0.92±0.008 in the unaffected hemisphere of patients (n=23), 0.92±0.001 in the affected side if collateral supply with blood was intact (n=16). There was no statistical significance between these groups. Saturation in affected hemispheres with impaired collateral blood supply (n=9) was 0.81±0.028, which was significantly different from all other groups (P<0.05, one way-ANOVA). We conclude, that transcranial pulse oximetry can detect local hypoxia if collateral blood supply fails. © 2002 Elsevier Science Inc. All rights reserved.