Doppler flow velocities in the middle cerebral artery of children with blocked cerebrospinal fluid shunts show changes that can be used as a non-invasive method for assessing shunt function. The velocities also change with age, and the purpose of this study was to define a reference range of values for children with controlled hydrocephalus. Flow velocities were measured in 248 asymptomatic children with shunts and an age-reference range was derived. Twenty children were subsequently admitted with suspected shunt malfunction and 11 of these required a shunt revision. On admission 9 of the 11 children requiring operation had Doppler values outside the reference range, whereas none of the children whose symptoms settled without surgery had abnormal values. The reference range will facilitate the use of transcranial Doppler for the assessment of children with suspected shunt malfunction.