It has been suggested that the presence of lacunar skull deformity in infants with myelomeningocele and hydrocephalus is indicative of later lowered intelligence and therefore is a useful criterion in early selection for treatment. In the present study, retrospective investigations were made of the clinical records and skull X-rays of 169 surviving children who had been admitted immediately after birth with myelomeningocele and hydrocephalus to the Children's Hospital, Sheffield. Adequate X-rays were available for 131 of the children, of whom 97 per cent exhibited some degree of lacunar skull deformity. At five years of age or older, the 131 children were tested on the Wechsler Intelligence Scale for Children. The differences in mean IQ between the various degrees of severity were not large, and it was found that 82 per cent of children with the most severe degree of lacunar skull deformity had IQs above 70. These findings do not support the use of lacunar skull deformity as an important measure in selection for treatment of children with myelomeningocele and hydrocephalus.