A 2-year-old girl with alobar holoprosencephaly associated with facial abnormalities, central diabetes insipidus, and a neuronal migration disorder is reported. The diagnosis of diabetes insipidus was based on low urine osmolality and low plasma ADH concentration during a water deprivation test, and clinical and biochemical improvement after desmopressin acetate administration. Because the posterior portion of the pituitary was located in the sella turcica and the hypothalamo-pituitary stalk was intact, the diabetes insipidus was presumed to have been caused by hypothalamic osmoreceptor dysfunction. MRI findings were compatible with alobar holoprosencephaly. In addition, heterotopic gray matter was recognized as a continuous band over a single ventricle. Defective cleavage of the prosencephalon associated with a neuronal migration disorder is characteristic of alobar holoprosencephaly.