Aim: To determine the prevalence, aetiology and clinical outcome in children w ith surgically treated hydrocephalus. Methods: A population-based study of all 208 liveborn children with hydrocephalus, 124 with infantile hydrocephalus and 8 4 with hydrocephalus associated with myelomeningocoele, born during 1989-1998 i n western Sweden. Aetiological and clinical information was collected from recor ds. Results: The prevalence of hydrocephalus was 0.82 per 1000 live births, 0.49 for children with infantile hydrocephalus and 0.33 for children with myelomenin gocoele. The prevalence of infantile hydrocephalus decreased during the period f rom 0.55 to 0.43 per 1000. In this group, the aetiology was prenatal in 55%and peripostnatal in 44%of the children. The origin was perinatal haemorrhage in al l cases born very preterm. The mortality rate was 5%for children with either in fantile hydrocephalus or myelomeningocoele. Mental retardation, cerebral palsy a nd epilepsy were significantly more frequent in the group with infantile hydroce phalus: 46%vs 16%, 31%vs 4%and 31%vs 10%, respectively. All children with infantile hydrocephalus born very preterm had at least one of these impairments, as did 80%of those with overt hydrocephalus at birth. Conclusion: A slightly d ecreasing trend for infantile hydrocephalus was observed during the 10-y period . Children with infantile hydrocephalus had a worse outcome than those with myel omeningocoele. The need for neurosurgical revisions for two-thirds of the child ren indicates the need for further development of prevention and treatment strat egies. Aim: To determine the prevalence, aetiology and clinical outcome in children w ith surgically treated hydrocephalus. Methods: A population-based study of all 208 liveborn children with hydrocephalus, 124 with infantile hydrocephalus and 8 4 with hydrocephalus associated with myelomeningocoele, born during 1989 -1998 in western Sweden. Aetiological and clinical information was collected from recor ds. Results: The prevalence of hydrocephalus was 0.82 per 1000 live births, 0.49 for children with infantile hydrocephalus and 0.33 for children with myelomenin gocoele. The prevalence of infantile hydrocephalus decreased during during the period f rom 0.55 to 0.43 per 1000. In this group, the aetiology was prenatal in 55% and peripostnatal in 44% of the children. The origin was perinatal haemorrhage in al l cases born very preterm. The mortality rate was 5% for children with either in fantile hydrocephalus or myelomeningocoele. Mental retardation, cerebral palsy a nd epilepsy were significantly more fr equent in the group with infantile hydroce phalus: 46% vs 16%, 31% vs 4% and 31% vs 10%, respectively. All children with infantile hydrocephalus born very preterm had at least one of these impairments, as did 80% of those with overt hydrocephalus at birth. Conclusion: A slightly d ecreasing trend for infantile hydrocephalus was observed during the 10-y period. Children with infantile hydrocephalus had a worse outcome than those with myel omeningocoele. The need for neurosurgical revisions for two-thirds of the child ren indicates the need for further development of prevention and treatment strat egies.