A system is presented for the classification of chronic herniations of the cerebellar tonsils in the absence of space-occupying intracranial lesions, based on a survey of the literature and 13 own cases. The Arnold-Chiari malformation in adults typically involves herniation of the cerebellar tonsils instead of herniation of the vermis as is typical when it occurs in infancy. Identification of chronic tonsillar herniation with the Arnold-Chiari malformation in adults was thought to require at least one other sign of the Arnold-Chiari complex, e.g. a medullary deformity. Cases for which chronic herniation and sclerosis of the cerebellar tonsils present as the only nervous lesion are classified as "chronic tonsillar herniation". Such cases may manifest with neurological symptoms during adult life, or they may be found incidentally at autopsy. Attention is drawn to the occurrence of chronic tonsillar herniation in 7 infants and children where it apparently represents a cause of sudden unexpected death. The overlap between chronic tonsillar herniation and the Arnold-Chiari malformation of adults is discussed in regard to the frequence of associated osseous anomalies, hydrocephalus and syringomyelia.