Descending cerebral transtentorial herniation (DTH) is a serious and often fatal complication of intracranial mass lesions. The condition can be inferred from clinical neurologic signs, but has not been visualized during life. Using midsagittal magnetic resonance images (MRIs), we compared vertical brainstem position on 50 images from normals and 21 images from 15 clinically stable patients with large supratentorial tumors. The length of Twining's line (T), the perpendicular distance from T to the pontomesencephalic junction (T-PMJ), and from T to the apex of the midbrain aqueduct (T-A) were measured. We also measured lateral shifts of the diencephalon and midbrain on axial images. T-PMJ decreased from 2.04 +/- 0.06 mm in normals to 0.94 +/- 0.2 mm in patients with large cerebral tumors (p less than 0.0001). Similarly, T-A decreased from 6.35 +/- 0.13 mm in normals to 4.83 +/- 0.35 mm in patients (p = 0.001). Lateral diencephalic-midbrain shifts often accompanied DTH but to an unpredictable degree. Either lateral or downward brainstem shift could occur alone and did not necessarily produce specific neurologic signs or an altered state of consciousness. Anatomic DTH occurs in life, it can be quantified with MRI, and in slowly developing cerebral mass lesions the process can precede the appearance of neurologic signs and symptoms that indicate lower-diencephalic or midbrain dysfunction.