We propose the use of near-infrared spectroscopy (NIRS) to monitor neonatal brain temperature non-invasively and continuously during hypothermic treatment by measuring changes in the NIR water absorption spectrum. Accurate measurements of the water spectrum in the range 650-1050 nm between 42 and 32 degree(s)C have been made and calibrated using a principal component regression, giving a standard error of calibration of <0.1 degree(s)C for 3 or more principal components. In vivo attenuation spectra measured on the adult forearm as a function of temperature have been employed in a preliminary investigation. In order to predict tissue temperature from changes in the water spectrum, we must first account for the effects of light scattering and haemoglobin absorption. A non-linear difference technique based on diffusion theory has been used to reconstruct the total absorption for each tissue spectrum, with respect to an assumed initial state. Second derivative spectroscopy was used to estimate the initial tissue chromophore concentrations for the reference spectrum. A linear fit to the reconstructed absorption spectra was then performed, using the NIR chromophore spectra and the water loading vectors determined from the PCR calibration. Tissue temperature was predicted with a standard error of approximately 1 degree(s)C.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.