To determine the contribution of extracranial oxygenation on regional cerebral oxygenation measured by an Invos 3100 near-infrared spectrometer, we measured oxygenation in blood drawn from both the facial vein (draining substantially blood from forehead areas) and the jugular venous bulb. There was no correlation between regional cerebral oxygenation and facial vein oxygenation (p = 0.35) but there was a significant correlation between regional cerebral oxygenation and jugular venous bulb oxygenation (p = 0.027). Linear regression analysis predicted a 3.6% change in regional oxygenation for every 10% change in jugular venous bulb oxygenation. We showed that extracranial tissue oxygenation had a negligible influence on the values recorded using near-infrared spectroscopy. Individual changes in jugular venous bulb oxygenation were poorly reflected. Data obtained by this near-infrared spectroscopy device are an unreliable guide to the adequacy of cerebral oxygenation.