Cranial computed tomography permitted an in vivo classification of brain edema. The distinction of the four basic types of edema (vasogenic, ischemic, cytotoxic, and periventricular) was important in establishing the chronicity, extent, and pathologic basis of cerebral abnormalities. Vasogenic edema (neoplasm, abscess) was readily distinguished from cytotoxic edema by the predominant involvement of the white matter and associated abnormal enhancement following intravenous contrast medium injection. The diminished density of ischemic edema was visualized in both gray and white matter and occurred in a vascular distribution. An absence of enhancement in the cerebral capillary bed and attenuation or nonvisualization of the circle of Willis following the rapid bolus injection of iodinated contrast medium defined an arrest of cerebral circulation as might be seen in brain death. Periventricular interstitial edema was most often a marker of acute or subacute obstructive hydrocephalus.