We report nine consecutive children and adolescents [five females and four males; aged 2 yr 8 months (m) to 18 yr 1 m] studied over the last 5 yr with idiopathic central diabetes insipidus. In addition to vasopressin deficiency, anterior pituitary hormone deficiencies were detected, either on evaluation at presentation or during follow-up studies over the following 3 yr. Four patients had an increased concentration of plasma PRL. One patient had multiple pituitary hormone deficiencies at diagnosis, and two others developed the same by 21 m of follow-up. Brain magnestic resonance imaging scans, performed at presentation, were originally interpreted as normal in four of nine patients, except for absence of the bright posterior pituitary signal; after retrospective review, two of nine were considered normal. All of the brain magnetic resonance imaging (MRI) scans showed positive findings by 14 m of follow-up. The first abnormal finding in all patients was isolated pituitary stalk thickening. Evaluation of cerebrospinal fluid (CSF) for hCG was positive in three of eight evaluated patients; the three positive CSF values were found at presentation and 3 and 9 m after presentation. All eight patients assessed were negative for CSF alpha-fetoprotein and cytology, and no patient had serum tumor markers. Transsphenoidal biopsy of the lesion in seven of nine patients showed a germinoma in six patients and inflammatory cells in one. The six patients with documented germinoma comprise 31% of the intracranial germinomas diagnosed in this age group at the University of California-San Francisco during the last 5 yr. The patient with mononuclear inflammatory cells on biopsy along with one other patient have had spontaneous resolution of their stalk thickening. So-called "idiopathic" central diabetes insipidus warrants close follow-up to determine the etiology, especially if anterior pituitary hormone deficiencies are detected. Normal brain MRI scans or scans that show isolated pituitary stalk thickening merit follow-up with serial contrast enhanced brain MRI for the early detection of an evolving occult hypothalamic-stalk lesion. CSF evaluation is recommended at presentation because elevated CSF hCG may precede MRI abnormalities.