To evaluate the risk of tonsillar herniation, the caudal pole of the cerebellar tonsil in posterior fossa mass lesions and anomalies was investigated with magnetic resonance imaging. The cerebellar tonsil was clearly demonstrated in relation to the surrounding structures on T1-weighted images in the paramedian or midline sagittal plane. In 78% of a control group of 50 cases, the tonsil was located just on or close to the upper border of the posterior lip of the foramen magnum. It was close to the line of the foramen magnum in 4 cases (8%), but displacement below this line was not observed in any case. In 18 cases of posterior fossa mass lesions, the tonsil tended to be in a lower position than in the control group, and tonsillar displacement below the foramen magnum was noted in 3 cases. Two of these cases had anterior medullary displacement with loss of the cerebrospinal fluid (CSF) space anterior to the tonsil; these patients had either rapid clinical deterioration or severe papilledema. In the remaining case and in 3 additional cases of Chiari malformation, the tonsils were displaced into the cervical canal with preservation of the CSF space, and the patients showed no signs of rapid clinical deterioration or increased intracranial pressure. The present study suggests that tonsillar displacement through the foramen magnum with associated anterior medullary displacement and loss of the CSF space anterior to the tonsil can be regarded as a sign of tonsillar herniation.