The development of computer-assisted tomography (CT) by Godfrey N. Hounsfield, has provided physicians with a noninvasive radiologic technique that has already revolutionized the practice of neuroradiology and the sister disciplines of neurology and neurosurgery. The accompanying article by Penn et al (p 1154) describes the use of CT scanning to determine blood volume in the brain. An expansion of their work will, one hopes, provide important new information about a variety of neuropathological states. Brain neoplasms, infarctions, cerebral edema, abscesses, and hemorrhages can now be diagnosed and studied by serial CT scanning with little danger or inconvenience to the patient. Although this technique cannot define basilar brain tumors with the accuracy of pneumoencephalography or describe in minute detail the vascular abnormalities of aneurysms or small arteriovenous malformations (as accomplished by arteriography), its overall delineation of intracerebral pathologic characteristics has an accuracy that exceeds isotopic scanning and a wealth of detail