Current concepts of brain herniation have depended largely on correlating clinical signs and symptoms with indirect radiographic studies and the results of postmortem neuropathology. This article describes measurements on midsagittal magnetic resonance imaging (MRI) scans that distinctly define normal and abnormal rostral-caudal relationships between the diencephalic-mesencephalic junction and the plane of the tentorial incisura, herein termed the incisural line. We similarly provide quantitative MRI scan measurements relating the cerebellum and the plane of the foramen magnum, termed the foramen magnum line. Measurements from 156 midsagittal and 63 coronal MRI scans performed on 123 normal adults, placed the iter of the aqueduct 0.2 +/- 0.8 mm (mean +/- SD) below the incisural line and the cerebellar tonsils 0.1 +/- 2.1 mm below the foramen magnum line. Defining 2 SD from these norms as abnormal, 23 patients with intracranial mass or obstructive lesions showed 4 distinct patterns of brain herniation, i.e., upward or downward transtentorial shift with or without accompanying cerebellar tonsillar herniation. Five patients with posterior fossa masses demonstrated displacement of the iter above the incisura ranging from 1.6 to 6.3 mm. Eighteen patients with supratentorial masses demonstrated displacement of the iter ranging from 2.0 to 11.0 mm below the incisura. Two-thirds of patients with upward and one-half of those with downward transtentorial shift had concurrent tonsillar herniation. In acute illnesses, MRI scan changes anticipated or confirmed clinical signs of brain herniation. In chronic cases, clinical and MRI scans correlated less well, with MRI sometimes revealing major degrees of anatomical herniation well in advance of clinical abnormalities.