Idiopathic hypothalamic dysfunction is a rare but well-defined entity in childhood characterized by adipsia-hypernatremia, obesity, poor thermoregulation, and disturbance of pituitary function. Two cases of idiopathic hypothalamic dysfunction are described. There are 10 previously reported cases in the literature, and the clinical features are compared. The present cases are unique in that the patients also had bilaterally dilated unresponsive pupils. In the first case, there was no demonstrable pathology at autopsy; in the second case, lymphocytic infiltration of the hypothalamus and midbrain associated with neuronal loss was present at autopsy. Possible etiologies are discussed.