The time taken for an extremely short pulse of near-infrared laser light to traverse the heads of 6 preterm infants was measured after death. The values obtained were used to calculate a differential path length factor (DPF), defined as the mean distance travelled by the photons divided by the distance between the points where light entered and left the head. The DPF was found to be 4.39 +/- 0.28. Knowledge of this factor will permit accurate quantitative measurements to be made by near-infrared spectroscopy of a range of indices of cerebral oxygenation and haemodynamics in live infants.