The clinical and neuroradiological findings in five patients with Chiari Type 1 malformation presenting symptoms in adult life are presented. New posterior fossa ratio methods have been applied to this material. The posterior cranial fossae are demonstrated to be small, and the most significant deviation from normal gave the ratio h/Tw: posterior fossa height in proportion to Twining's line. The implications of these findings are discussed in relation to the static and dynamic factors that are operating in the development of neurological symptoms in Chiari type 1 malformation. A final discussion is given comparing the Chiari type 1 malformations to meningo-myelocele. The findings using posterior fossa ratio methods may strengthen the indications for suboccipital craniectomy in these patients.