Magnetic resonance imaging was used to define quantitatively the position of the cerebellar tonsils in the normal population and in patients with Chiari malformations. The average distance of the tonsillar tips from the foramen magnum was 2.9 +/- 3.4 mm above the foramen in 82 subjects without posterior fossa abnormality or increased intracranial pressure, and 10.3 +/- 4.6 mm below the foramen in 13 patients with Chiari malformations (p less than 0.005). Consequently, extension of the tonsils below the foramen magnum is considered normal up to 3 mm, borderline between 3 and 5 mm, and clearly pathologic when it exceeds 5 mm.