To find a suitable method for measuring regional cerebral blood flow (rCBF) rapidly at the bedside is still a matter of investigation. The purpose here was to develop a noninvasive method for bedside rCBF measurement and to validate it with a standard method such as perfusion-weighted magnetic resonance imaging (MRI). In 11 healthy volunteers 44 measurements with near-infrared spectroscopy (NIRS) and perfusion-weighted MRI without and with a mean continuous positive airway pressure (CPAP) of 10 mbar were carried out. Four (NIRS) optodes were placed bilaterally on the forehead and 25 mg indocyanine green (ICG) was injected. New algorithms were developed to calculate rCBFNIRS and rCBVNIRS. In 6 volunteers data analysis was successful. No complications associated with the method were observed. During CPAP breathing rCBFNIRS decreased from 18.5 + 6.9 16.1 + 6.2 ml/100 g/min (P = 0.034). Mean values for rCBFMRI decreased from 256 +/- 90 to 216 +/- 62 ml/100 g/min (P = 0.012). Bland and Altman plots showed that the differences did not vary in any systematic way over the range of rCBF or rCBV values assessed and 100% of differences were within the interval mean +/- 2 SD of differences. Limits of agreement (mean +/- 2 SD) were +/- 76.4 ml/100 g/min for rCBF and +/- 15.6 ml/100 g for rCBV. The NIRS ICG dye dilution technique is a promising method for serial noninvasive bedside CBF measurements. The preliminary data indicate that measurements are in agreement with values obtained by perfusion-weighted MRI.