Detailed linear, angular and surface area measurements were undertaken on the lateral skull radiographs of 32 patients with proven idiopathic adult Chiari type 1 malformation. Basal angles were larger, the clivus was shorter and Klaus' index was reduced in patients compared to normal controls. The size of the posterior fossa was smaller in patients than in controls. By discriminant analysis of the skull X-ray data it was possible to identify correctly two thirds of patients as belonging to the patient group. The findings indicate that craniocervical bony anomalies in the adult Chiari malformation are commoner than was previously realised, and they support the view that tonsillar herniation in these patients results from bony occipital dysplasia.