The authors experienced 3lpatients with meningioma since CT scans had become available. There were various extents of low density areas surrounding the tumors on CT scans. These diminished density areas were thought to be related to tissue necrosis, demyelination, widening of the subarachnoid spaces, or cerebral edema. A finger-shaped extension within the white matter is generally regarded as cerebral edema. On the other hand, meningiomas were thought to be associated with different degrees of brain edema in their neighborhood, and the reason for the difference of edema extent in each case was unknown. So a comparative study between the CT findings of meningiomas and the angiograms was performed to clarify the mechanism of expansion of cerebral edema around these tumors. The conclusions are as follows: 1) there is no definite relationship between the perifocal low density and the size of tumors or the pathological findings; 2) tumors that exhibit high vascularity or block the venous drainage in angiograms tend to show greater perifocal low density areas; 3) intraventricular or parasellar meningiomas tend to show smaller low density areas; and 4) it is reasonable to assume that the degree of edema surrounding a meningioma is defined by a factor in which the arterial, venous, and CSF systems are correlated to each other.