The classification of the developmental malformations of the posterior fossa associated with hydrocephalus proposed by Chiari is systematic and comprehensive. It is based upon the relative position of cerebellum and brainstem in relation to the foramen magnum and the upper cervical canal. Types I to II are degrees of a similar abnormality in which a conical deformity of the posterior midline cerebellum and the elongated brainstem lie at or below the foramen magnum. In the Type IV deformity, in contrast, the cerebellum and brainstem lie fully within the posterior fossa and their position in relation to the foramen magnum is normal. The most common example of this type is probably the Dandy-Walker malformation, in which a defect in the inferior vermis is congruent with a ventriculocele of the enlarged fourth ventricle. The Chiari malformations are generally associated with hydrocephalus. The most comprehensive and coherent theory of the origin of these malformations is that abnormal hydrostatic forces, acting principally during the second and early third trimesters of gestation, provide the mechanism of posterior fossa deformity. Morphological evidence indicates that posterior fossa structures in Types I to III are "pushed down" from above. Radio-isotope dilution studies suggest that the mechanism of hydrocephalus in Types I to III is overproduction of cerebrospinal fluid (CSF); initially there may be no obstructive component. In those cases without spina bifida, ventricular enlargement should occur early and be greatest in the forebrain, driving the tentorium and posterior fossa structures downward. Where there is a fistulous communication between the ventricular system and the amniotic cavity via a spina-bifida defect, a pressure gradient between intracranial and spinal compartments may be sufficient to account for downward displacement of posterior fossa structures. In this instance ventricular enlargement may occur only after cerebellar impaction causes frank obstruction to the flow of cerebrospinal fluid. In the Dandy-Walker malformation, the mechanism of hydrocephalus is probably obstruction to flow of cerebrospinal fluid through the roof of the fourth ventricle. Enlargement of the fourth ventricle is sufficient not only to compress the cerebellum and give rise to the inferior vermal defect, but also to offset forces above the tentorium and prevent its caudal displacement from the parietal region.