CT findings and pathological features of 102 supratentorial astrocytomas were evaluated, and the surgically removed specimens of 10 cases studied by electron microscopy. Five pathological indices-anaplasia, vascularity, interstitial water content, necrosis and tumor fibers were graded from 1 to 4, and studies by the double-blind method. CT scans were divided into 4 groups according to attenuation coefficients and contrast enhancement. The data were statistically analysed. The results showed that the interstitial water content and tumor fibers were statistically different in low-density lesions and isodensity or hyperdensity lesions. The peritumoral low-density specimens correlated well with infiltrated tumor cells, which had swollen mitochondria. No definite correlation was found between the histological subtypes of astrocytomas and CT images. These findings suggest that high water content and lower physical density are responsible for the low-density of astrocytomas. Abnormalities of capillary ultrastructure result in the accumulation of contrast medium. The peritumoral area of low-density seen on CT scans is the real extent of the edematous tumor. But it is impossible to distinguish the histological subtypes of astrocytomas by CT findings.