Continuous wave near-infrared spectroscopy (NIRS) measurements of cardiac correlated changes in attenuation in the adult human head were computed using a Fourier analysis technique that eliminates the positive error bias associated with the magnitude of the Fourier coefficient. These attenuation changes were used to determine wavelength dependence of differential pathlength, DP(lambda), at four stages during progressive hypoxia (21, 17, 13 and 9% FIO2) in normal volunteers. The effects of incorporating DP(lambda) into NIRS algorithms to compute relative concentration changes and absolute concentration of oxyhaemoglobin and deoxyhaemoglobin are discussed. Because variations in DP(lambda) are restricted to wavelengths below 780 nm, absolute concentration calculations are influenced by hypoxia-induced changes while relative concentrations are unaffected. However, even accounting for changes in DP(lambda) did not allow computation of physiologically reasonable absolute concentrations of the haemoglobin species.