Routine CT scanning in 221 patients with craniosynostosis revealed ventricular dilation in 40. In 5 hydrocephalus was obviously unrelated to the craniostenosis. The remaining 35 cases were associated almost exclusively with syndromic craniosynostosis. Ventricular dilation was mild in 22, moderate in 9, and marked in 4 patients. Clinical and radiological findings strongly suggest that three different mechanisms are involved in the pathogenesis of hydrocephalus: primary cerebral maldevelopment, brain atrophy, and CSF outflow obstruction. In the diagnosis of hydrostatic hydrocephalus with craniosynostosis, head circumference is no indicator of progressive hydrocephalus, and intracranial hypertension may be due either to CSF accumulation or to craniostenosis. The present study indicates that shunt treatment prior to correction of synostosis should be restricted to a few cases of rapidly progressing hydrocephalus. Secondary shunting of hydrocephalus may be considered if intracranial pressure remains high despite adequate cranial decompression. Shunting is not an appropriate treatment for craniostenosis--even in cases of concurrent ventricular dilation.