Chiasmatic/hypothalamic gliomas usually are histologically benign astrocytomas that may recur many years after diagnosis and treatment. Three children with chiasmatic/hypothalamic gliomas who were treated at the authors' institution returned 9.5, 11.5, and 2 years, respectively, after radiation therapy (RT) because visual and neurologic deterioration developed. Neuroradiographic studies, including arteriography in two of the patients, showed large mass lesions. These were presumed to be recurrence of tumor, and chemotherapy was administered. Pathologic examination of two children who died and of the third who had a biopsy revealed only a minimal amount of residual, histologically benign astrocytoma, whereas the bulk of the specimen consisted of numerous vessels of variable size. These probably represented incorporation of the rich vasculature in the chiasmal region into the tumor, which underwent degeneration secondary to RT. Radiographic methods did not distinguish progressive tumor growth from the vasculopathy and led to inappropriate clinical diagnoses and treatment.