The mean cerebral blood flow (mCBF) has been studied in 40 patients with brain tumors undergoing radiation and chemotherapy, and with ages ranging from 20 to 69 years, using 133Xe and single photon emission CT. A comparison was made of mCBF of 40 normal volunteers (34 males and 6 females) whose ages ranged from 25 to 82 years. The results were as follows: Some patients exhibited increased mCBF during external irradiation therapy. Over 3 months after the initiation of external irradiation, the average mCBF decreased significantly, ranging from 5.0 to 8.8 ml/100g/min, compared to the control group. A regression equation, which was obtained using multiple regression analysis, was delta mCBF = -3.140 + 0.181 A - 0.069 E - 0.022 V - 0.543 C (delta mCBF: subtract measured mCBF from presumed mCBF at the same age, A: age, E: dose of external irradiation, V: volume of lesion, C: CSF-cranial ratio, multiple regression coefficient: r = 0.536, P less than 0.002). This result means that the mCBF of non-affected hemispheres decreases in direct proportion to the volume of the lesion, degree of brain atrophy, and dose of external irradiation. In this study, the tendency for mCBF to decrease in direct proportion to these factors was found to be more pronounced among young patients. There were no side effects caused by the additional irradiated doses intraoperatively or caused by residual tumors on the mCBF of non-affected brains. Also, there was no significant difference in the average mCBF between cases in which whole and local brain irradiation was employed, as well as between cases in which there were different histopathological diagnoses.(ABSTRACT TRUNCATED AT 250 WORDS)