The application of transcranial Doppler ultrasound to the diagnosis of blocked ventriculo-peritoneal shunts was studied in 63 children. Thirty two of these required shunt revision, whereas in 31 children symptoms resolved without surgery. The group of children requiring shunt revision had a significantly higher mean Gosling pulsatility index, than both the group of children whose symptoms resolved and a group of age-matched controls (p less than 0.001). Those with a raised pulsatility index were more likely to have higher intracranial pressure. There was no correlation between CT scan changes and the pulsatility index.