Using the transcranial Doppler technique to investigate the postnatal and developmental changes of cerebral blood flow in newborn infants, we found that the systolic and mean blood flow velocities increased after birth. A rapid increase in blood flow velocities was demonstrated during the first several days. Pulsatility index, which was initially high, gradually fell after birth and remained stable thereafter. The blood flow velocities of small-for-date infants were determined by their gestational ages. The systolic and mean blood flow velocities were influenced more by postnatal age than by gestational age, birth weight, or body weight. The increase in conceptional age was associated with a decrease in the pulsatility index and an increase in the systolic and mean flow velocities. The transcranial Doppler technique is a useful method and will provide more accurate information about the cerebral hemodynamics in neonates than the previously employed continuous wave Doppler technique.