Transcranial Doppler ultrasonography (TCD) was performed on 14 patients with hydrocephalus (age range 1 day to 12 years old) before and after ventriculoperitoneal shunting. TCD was also performed with simultaneous intracranial pressure (ICP) measurements during ventricular taps through a reservoir in 7 patients. Measurements of the resistance index (RI) = (S-D)/S, peak systolic (S), enddiastolic (D) and time-averaged mean flow velocities were made. After ventricular taps and ventriculoperitoneal shunting there was a significant decrease in RI in all patients. This was due to a greater increase in D compared to S, which suggests a decreased distal cerebrovascular resistance. There was a significant correlation between RI and ICP in the older infants and children and in individual neonates. Successful cerebrospinal fluid diversion reduces ICP and cerebrovascular resistance, thus improving cerebral perfusion. The RI is a reliable index for serial monitoring of cerebrohaemodynamic change in patients with hydrocephalus.