The computed tomography (CT) scans of nine patients with vasogenic cerebral edema and five monkeys with cryogenic lesions were correlated with histologic findings. The areas of diminished density on the CT scan corresponded to foci of histologically recognizable edema. The attenuation number of the edematous tissue in the human varied from 18.0 to 28.8 Hounsfield units (HU: 1.000 scale) and in the monkey from 27.0 to 34.7 HU. There was a rough correlation between the extent of decrease in the attenuation number and the degree of staining of edematous tissue by the periodic acid-Schiff method. The decreased attenuation number of edematous tissue signifies dilution by edema fluid only when it is above 20 HU or represents a change of less than 12 HU. Lower numbers imply an additional process, possibly transformation of complex into neutral lipids. It is our belief that cerebral edema in the human can be quantitatively studied with the presently available CT scanners.