The computed tomography (CT) scans of 92 infants and children were reviewed. Thirty-three had proven atrophy, and 44 had proven obstructive hydrocephalus. Twelve CT measurements were made, and the results were subjected to computer analysis. The pathologic patients were divided into three groups by ventricular size index: mild (33--39%), moderate (40--46%), and severe (greater than 46%) enlargement. Obstructive patients showed much greater measurements for the temporal horn diameter and the frontal horn radius. The angle of the frontal horn was narrower in the obstructed group than in the atrophic patients. As expected, there were significantly more sulci visualized in the atrophic group than in the obstructed group. Three of these four parameters were directly related to concentric expansion of the ventricles in the presence of obstructive hydrocephalus. This is contrasted with passive dilatation of the ventricular system with preservation of the normal ventricular configuration in atrophy. The temporal horn diameter, frontal horn radius, the angle of the frontal horn, and the number of sulci all distinguish between obstruction and atrophy with individual probabilities of less than 0.05. When used together, they assure a high probability of separating obstruction from atrophy. Preliminary data suggest that these findings may have application in adults, but the number of confirmed observations has been too small for statistical analysis.