Purpose : To prospectively evaluate pediatric patients with localized primary brain tumors for evidence of endocrinopathy before radiotherapy (RT). Methods and Materials : Seventy-five pediatric patients were evaluated with the arginine tolerance test and l-dopa test for growth hormone secretory capacity and activity; thyroid-stimulating hormone surge and thyrotropin-releasing hormone stimulation test for the hypothalamic-thyroid axis; the 1-μg adrenocorticotropin hormone (ACTH) and metyrapone test for ACTH reserve; and, depending on age, a gonadotropin-releasing hormone stimulation test to determine gonadotropin response. The study included 38 male and 37 female patients, age 1–21 years with ependymoma ( n = 35), World Health Organization (WHO) Grade I–II astrocytoma ( n = 18), WHO Grade III–IV astrocytoma ( n = 10), craniopharyngioma ( n = 7), optic pathway tumor ( n = 4), and germinoma ( n = 1). Seven patients receiving dexamethasone at the time of the evaluation were excluded from the final analysis. Results : Of 68 assessable patient, 45 (66%) had evidence of endocrinopathy before RT, including 15 of 32 patients (47%) with posterior fossa tumors. Of the 45 patients, 38% had growth hormone deficiency, 43% had thyroid-stimulating hormone secretion abnormality, 22% had an abnormality in ACTH reserve, and 13% had an abnormality in age-dependent gonadotropin secretion. Conclusion : The incidence of pre-RT endocrinopathy in pediatric brain tumor patients is high, including patients with tumors not adjacent to the hypothalamic-pituitary unit. These data suggest an overestimation in the incidence of radiation-induced endocrinopathy. Baseline endocrine function should be determined for brain tumor patients before therapy. The potential for radiation-induced endocrinopathy alone cannot be used as an argument for alternatives to RT for most patients. Pre-RT endocrinopathy may be an early indicator of central nervous system damage that will influence the functional outcome unrelated to RT.