Near-infrared spectroscopy is a technique used to monitor cerebral oxygenation. To validate the method, we measured regional oxygen saturation (rSo2) in the brains of 18 dead subjects (mean age, 74.4 ± 14.6 years) 19.8 ± 18.2 h (range, 1–73) after cessation of systemic circulation, and in 15 healthy probands (mean age, 34.2 ± 8.7 years) with an INVOS 3100 cerebral oximeter. The mean (±SD) rSo2in the dead subjects was 51.0 ± 26.8% [range, 6–88%; left, 48.4 ± 28.0% (n= 21); right, 54.4 ± 25.7% (n= 16)]. The mean rSo, in the control group was 68.4 ± 5.2% (range, 60–76%; left, 68.1 ± 5.0%; right, 68.7 ± 5.6%). After removal of the brain at autopsy in five of the dead subjects, the rSo2was 73.4 ± 13.3% (15 measurements). Six of 18 of the dead subjects had values above the lowest values found in the healthy adults (≥60%). These findings raise concerns about the validity of cerebral rSo2data in adults obtained by the INVOS 3100 system.