The occurrence of behavioural problems in a population-based series of children with infantile hydrocephalus (non-spina bifida) was analysed, using parent questionnaires. Children with both infantile hydrocephalus and mental retardation had significantly more behavioural problems compared with those with no mental retardation and controls. Inattentiveness and hyperactivity were particularly typical. No differences were found between children with infantile hydrocephalus and no mental retardation and the control group.