In up to 25% of cases of children with central diabetes insipidus no organic cause can be documented. We present three boys (age 2.2, 2.3 and 6 years at diagnosis) with acute onset central diabetes insipidus, in whom the only pathological finding using MRI was a thickened central part of the pituitary stalk (> 2.5 mm). Recent reports demonstrate similar MRI findings in adults with Langerhans cell histiocytosis (LCH), sarcoidosis, or tuberculosis, and in children with proven LCH and diabetes insipidus. In those adults with LCH, the pituitary stalk lesion has been histologically verified as a sequela of LCH. In contrast, in two of our three patients pituitary stalk thickening preceded the typical peripheral lesions of LCH by several months, whereas in the third patient there is as yet no evidence of systemic disease. We conclude that thickening of the central part of the pituitary stalk might represent the first manifestations of LCH clinically presenting with diabetes insipidus. MRI investigation of the pituitary stalk in children with unexplained central diabetes insipidus and accurate follow up in patients with thickening of the pituitary stalk in necessary to avoid missing other manifestations of a systemic disease.