Doppler ultrasound investigation of cerebral blood flow velocity was performed in hydrocephalic infants through the anterior fontanelle. Systolic (S) and end-diastolic (D) frequency values recorded on the anterior cerebral artery were used to define the pulsatility index (PI) calculated from the equation PI=S-D/S. Comparison between systolic, end-diastolic and pulsatility index values of 50 normal infants and 10 hydrocephalic infants showed a statistically significant difference (P less than 0.05) for systolic and pulsatility index values. However, no significant difference was found for end-diastolic values. The authors believe that the phenomenon could be explained as an increase of the cerebrovascular compliance which counteracts the increase of the perivascular pressure in an attempt to maintain a normal cerebral blood flow. Therefore, the transfontanelle Doppler ultrasound technique may provide a useful and early tool in diagnosing cerebral blood-flow changes in hydrocephalic infants.