Tumor blood-brain transfer constant of iopamidol (K) and plasma volume (Vp) were measured in 10 patients with primary brain tumors before and after 7 days of dexamethasone treatment (4 x 4 mg per oral per day) using X-ray computed tomography. Both K and Vp decreased significantly after dexamethasone treatment with p < 0.01 and 0.09 respectively according to one-tail paired t-test. The mean percentage decrease in K and Vp was 32% and 10% respectively. Functional images of the two parameters before and after treatment were generated and showed clearly the effect of steroids on the reduction of K in brain tumors. In contrast, when before and after treatment contrast enhanced CT scans were compared, no difference was observed in the enhancement in 8 out of 10 cases. The result obtained support the conclusion that: (1) corticosteroids reduce the blood-brain permeability to small hydrophilic molecules; (2) the X-ray computed tomography method we have developed can be used to measure the K and Vp response of brain tumors to steroid therapy; and, (3) ordinary contrast enhanced CT scans at one fixed time or multiple times after contrast injection are not sensitive in detecting the reduction of K due to steroids.