Because of its noninvasive property of direct imaging of the size and shape of the lesion, CT scan is significant not only for the diagnosis of the brain tumors, but also for the assessment of its chronological and/or therapeutic change in size. Contrary to the conventional cerebral angiography and contrast encephalography, CT scan is feasible to detect the tumor repeatedly at short intervals. From March 1976 to October 1977, 197 cases of brain tumors were examined by CT, 6 tumors (1 pineal tumor, 2 cerebellar vermian tumors and 3 supratentorial tumors) of which were radiated after the initial CT scan and re-examined by CT at every 400-1,000 rads radiation, and their grade of radiosensitivity could be evaluated. Immediately after the radiation of 400 rads for five days to the large pineal tumor of a 16-year-old boy, CT exactly showed the markedly diminished size of the tumor, and the tumor disappeared on CT after the 4,000 rads radiation. Cerebellar vermian tumors suspected of medulloblastoma were also diminished in size on CT after the 600 rads radiation. One supratentorial tumor was radiosensitive and disappeared at the end of radiation therapy. Other supratentorial tumors were not changed on CT during the radiation. Procedures for the therapeutic diagnosis, when combined with CT, can easily indicate whether the tumor is radiosensitive or not, without any invasive examination and/or operation.