Attention is drawn to the variation in the computed tomography (CT) attenuation value of water with temperature. A difference of 5 to 6 Hounsfield units (1,000 scale) between typical room and body temperatures can be predicted as a consequence of the change in the density of water and was confirmed experimentally. This difference may be of significance in the calibration of CT machines, in the determination of "absolute" CT attenuation values for body tissues and fluids, and in comparing tissue attenuation values obtained in vivo and in vitro.