Attention is drawn to the Arnold-Chiari malformation as a possible cause of internal hydrocephalus in spina bifida. The malformation was found in ten consecutive examples of lumbo-sacral or dorsi-lumbo-sacral meningo-myelocele. A comparable but far less pronounced malformation of the medulla oblongata is described in a case of lumbo-sacral meningocele without hydrocephalus. It is suggested that a more typical Arnold-Chiari malformation may be the cause of the hydrocephalus that complicates some examples of meningocele.