Remodelling the cranial vault in an attempt to increase the intracranial volume and thus control intracranial hypertension, whilst at the same time improving the patient's appearance, has been the mainstay of surgery for syndromic craniosynostosis. We report a case of craniosynostosis in whom cranial vault expansion was followed by the development of hind-brain herniation and hydrocephalus. This prompted a review of our other cases of craniosynostosis who had been evaluated by magnetic resonance imaging following surgery in order to assess the frequency of hind-brain herniation and hydrocephalus in these children. Magnetic resonance imaging had been performed in the postoperative evaluation of 34 cases of craniosynostosis who had undergone procedures intended to increase the intracranial volume. The position of the cerebellar tonsils and the presence or otherwise of hydrocephalus was recorded for all cases. The effectiveness of surgery in treating raised intracranial pressure (ICP) was evaluated by means of postoperative ICP monitoring and had been performed in 22 cases. Herniation of the hind-brain below the level of the foramen magnum was observed in 18 cases (53%). Hydrocephalus, requiring the insertion of a ventriculoperitoneal shunt, was present in 14 cases (41%) and had developed after the cranial vault procedure in 9. The mean sleeping ICP measured postoperatively was normal (<10 mm Hg) in 5, borderline (10-15) in 7, and raised (>15 mm Hg) in 10 cases. Cranial vault expansion in complex craniosynostosis may fail to address the underlying aetiology of intracranial hypertension. Furthermore, both hydrocephalus and hind-brain herniation may develop following such surgery. Neither the increase in intracranial volume afforded by cranial vault expansion nor the shunting of hydrocephalus precludes the persistence of abnormal ICP. These findings are discussed in the light of possible mechanisms, in addition to cephalocranial disproportion responsible for intracranial hypertension in complex craniosynostosis. The implications for the surgical management of complex craniosynostosis are reviewed.