The aim of the present study was to generate normal reference data for anterior and middle cerebral artery blood flow velocity and resistance index in preterm and term infants during the first 8 hours of life. The study population longitudinally included 120 healthy preterm and term infants (gestational age 24 to 41 weeks), all of appropriate weight for gestational age. The following parameters were studied: peak-systolic velocity, end-diastolic velocity, mean velocity, and resistance index. All parameters were measured in the anterior cerebral artery, in the left middle cerebral artery, and in the right middle cerebral artery with the use of Doppler colour ultrasonography. In addition, we studied the ratio of mean arterial blood pressure to mean velocity in the three cerebral arteries as a further estimate of cerebral relative vascular resistance. We found that cerebral blood flow velocities increased significantly with increasing gestational age and birthweight, both in the anterior cerebral artery and in the right and left middle cerebral arteries. Resistance index, both in the anterior cerebral artery and in the middle cerebral arteries, increased significantly only with increasing gestational age. Relative vascular resistance decreased significantly with increasing gestational age and birthweight in the three cerebral arteries. Significant differences were found (p<0.05) in these values between the anterior cerebral artery and the middle cerebral arteries. The narrow time frame (2 to 8 hours) that we used to evaluate cerebral blood flow velocity often represents a significant moment at which decisions are made that can be fundamental for the outcome of the newborn infant.