Near infrared spectrophotometry has been used to measure total cerebral hemoglobin concentration (micromol/l) as a major indicator of the oxygen transport capacity in neonates. The aim of this study was to find out how the position of the probe influences the quality of the measurement and the actual cerebral hemoglobin concentration-values. We studied 10 healthy preterm infants with a mean gestational age of 31.5 weeks and a birthweight of 1513 g. The data were collected by a two channel near infrared spectrophotometry system using a geometrical principle to measure absolute cerebral hemoglobin concentration. The incoming signal of the light emitting diode as a value allows a prediction of the quality of the measurement: a high value refers to a high signal/noise ratio. Starting from the centre of the forehead (0%) for each measurement the probe was moved by 2.5% of the headcircumference to the left respectively right side of the head up to 20%. The cerebral hemoglobin concentration-values increased from 87 respectively 93 micromol/l up to 164 respectively 173 micromol/l on the right respectively left side, while the light emitting diode signal-values decreased from 21 respectively 21 down to 10 respectively 11, the more laterally the probe was moved. There were two plateaus of these variables in the frontal (0-5%) respectively lateral (15-20%) region. A further investigation on a solid phantom for premature heads showed that hair has either no or a contrary effect on the cerebral hemoglobin concentration-values than expected. The extracerebral tissue (soft tissue, skull, cerebrospinal fluid layer) is discussed to have a significant influence on the light attenuation in adult heads. Still there is no evidence for a significant effect on prematures, because this overlying tissue is much thinner and more translucent than the one in adults. Absolute cerebral hemoglobin concentration measured by near infrared spectrophotometry is substantially influenced by the position of the probe at the infant's head. Considering our results we recommend placing the probe at 2.5% of the headcircumference away from the centre of the forehead for the measurement of cerebral hemoglobin concentration in premature infants.