We prospectively studied craniosynostosis, regardless of neurologic status, by cranial computed tomography or psychometric testing in 56 children. None of the 27 children with simple craniosynostosis (single or multiple suture involvement) had evidence of hydrocephalus on CT scan. Of the 24 patients with simple craniosynostosis who underwent psychometric testing, 17 were of average intelligence; six were in the low average range. The single mentally retarded child had a history of severe perinatal asphyxia. Hydrocephalus occurred more frequently (five of 23 cases) in children with complex craniosynostosis syndromes, including Pfeiffer syndrome, Crouzon syndrome, and kleeblattschädel deformity. More striking than hydrocephalus, however, was the finding of dysmorphic ventricular dilation in eight patients, including the three children with Apert syndrome and four with Crouzon syndrome. Nineteen of the 25 children with complex craniosynostosis syndromes receiving psychometric testing were of normal intelligence. Four children with borderline normal intelligence had either hydrocephalus or ventricular dilation. The two children with mental retardation were sisters with Crouzon syndrome whose family included other retarded individuals. This study indicates that the incidence of hydrocephalus and mental retardation in craniosynostosis is lower than reported previously.