Delayed effects of radiation therapy (RT) on twenty patients with germ-cell tumors, including 17 cases of germinoma, were evaluated with special reference to cerebral blood flow (CBF). All patients except for one of germinoma had craniotomy to verify the tumor histology, then RT followed with a total dose of 39.6 to 60 Gy for each patient. A follow-up study with CT scans and MRI proved not any recurrence of the tumors during the past 14-year period between June, 1975 and April, 1989. On the other hand, CBF was measured by 133Xe inhalation 1 to 14 years after RT. 40% of all patients were below critical values of CBF with higher frequency in the younger. Statistically, the post-irradiated changes of CBF were not related to a total administered dose, grading of brain atrophy, or physical mental developments, but closely related to irradiated volume dose (product of absorbed dose and field, rad.cm3) only for the age ranging from 8 to 15 years at RT. These results suggested that late effects of RT on CBF were more strongly depend upon absorbed dose and field of irradiation to brain in children than in adolescents.