A heterogeneous condition associated with other cerebral malformations, metabolic disorders, and degenerative diseases beginning in fetal life. It may be global or, less commonly, involves only the vermis; selective involvement of the lateral cerebellar hemisphere with preservation of the vermis also is described but is rare. Vermal hypoplasia may leave a gap in the cerebellar midline, as with the autosomal recessive Joubert syndrome, or may be absence of the vermis with fusion of the medial surfaces of the cerebellar hemispheres and dentate nuclei, a condition known as rhombencephalosynapsis. Global cerebellar hypoplasia sometimes occurs without other cerebral dysgeneses. The most constant clinical findings in infancy are gross motor developmental delay and generalized muscular hypotonia; inconstant findings include nystagmus, corticospinal tract deficits, mental retardation, and epilepsy. Cerebellar hypoplasia may result in histochemical changes in the muscle biopsy of fiber-type predominance or congenital muscle fiber-type disproportion, but necrotizing and dystrophic alterations are not found. CT scan can often confirm the diagnosis, but MRI with sagittal images demonstrates it best. The basis pontis also may be hypoplastic. Prognosis depends on underlying associated diseases, especially if progressive. Dysgeneses of the forebrain, including most of the primary disorders of neuroblast migration, may be associated CNS findings. Many genes are involved in this syndrome, and cerebellar hypoplasia is a feature in many chromosomal disorders.