Two cases of upward displacement of parts of the brain stem and the cerebellum are discussed. In both cases shunts for a hydrocephalus had been operating for a long time (three and a half years in one and seven months in the other). One infant had a meningomyelocele but no complete Arnold-Chiari malformation. The cause of the hydrocephalus in the other case was doubtful, but was thought to be transtentorial displacement of the brain stem and the cerebellum causing traction on the pons and consequent meningeal fibrosis with obstruction of the cisterna pontis. In this case osseous hypoplasia of the posterior fossa was excluded by craniometry, so the upward displacement must have been due to hypoplasia of the tentorium cerebelli and its low attachment to the occiput, bringingg about a narrowing of the infratentorial space. The authors believe the condition to be a primary malformation, and reject the hypothesis of secondary herniation following long standing supratentorial decompression. The concept of a primary malformation is supported by previous descriptions of the condition in cases with spina bifida, Arnold-Chiari malformation, and hydrocephalus without shunting, as well as by other features in the cases which have been recorded.