Cystic malformations in the posterior cranial fossa result from developmental failure in the paleocerebellum and meninges. The authors present the case of an infant with hydrocephalus associated with cystic dilation of the foramina of Magendie and Luschka. This 7-month-old female infant presented with sudden onset of tonic-clonic seizures. Computed tomography revealed tetraventricular hydrocephalus. Magnetic resonance imaging demonstrated a cyst communicating with the fourth ventricle and projecting to the cisterna magna and the cerebellopontine cisterns through the foramina of Magendie and Luschka. A suboccipital craniotomy was performed for removal of the cyst wall, and the transparent membrane covering the foramen of Magendie was removed under a microscope. After the surgery, the patient's hydrocephalus improved and a phase contrast cine MR imaging study showed evidence of normal CSF flow at the level of the third and fourth ventricles. Three weeks later, however, the hydrocephalus recurred. An endoscopic third ventriculocisternostomy was performed to address the possibility of stagnant CSF flow in the posterior cranial fossa, but the hydrocephalus continued. Finally the patient underwent placement of a ventriculoperitoneal shunt, resulting in improvement of her symptoms and resolution of the hydrocephalus. On the basis of this experience and previously published reports, the authors speculate that the cystic malformation in their patient could be classified in a continuum of persistent Blake pouch cysts. Hydrocephalus was caused by a combination of obstruction of CSF flow at the outlets of the fourth ventricle and disequilibrium between CSF production and absorption capacity.