Background and purpose: It is important to find a reliable and bedside method, which can estimate the cerebral blood flow (CBF) of patients in clinical settings. Estimation of CBF by calculating a blood flow index (BFI) using continuous wave near-infrared spectroscopy (CW-NIRS) and indocyanine green (ICG) as an iv tracer has been proposed to be a feasible and promising method. To validate if the BFI method can detect relative changes in CBF we compared data with the established method 133Xenon single photon emission computer tomography (133Xe-SPECT). Methods: Ten healthy subjects were investigated before and after a bolus of acetazolamide. NIRS data were obtained using a multi source detector separation configuration in order to assess a corrected BFI (BFIcorr) value, which attempts to eliminate contamination of skin blood flow. Results: Data obtained showed no significant correlation between CBF changes measured by 133Xe-SPECT and BFI corr (0.133, P = 0.732). After acetazolamide, a 49% increase in CBF was detected using the 133Xe-SPECT method, whereas no changes in any ICG variables were observed after acetazolamide. Conclusion: The study shows that it is not possible to obtain reliable BFI data, which reflect changes in CBF after acetazolamide infusion, using the CW-NIRS and ICG method.