To investigate at what age hydrocephalus is detected and to assess the role of head circumference measurements in detecting hydrocephalus, we performed a retrospective chart review in children with hydrocephalus treated in a tertiary paediatric hospital in the Netherlands. The study group contained 146 patients; 38 patients (31%) were referred because of abnormalities in head circumference. Eighty-nine per cent of the patients were detected in the first year of life. After this period, no patients were referred because of an abnormal head circumference. Therefore, head circumference measurements seem to have little value for detecting hydrocephalus after the first year of life.