We describe a three-year-old boy who had a growth and psychomotor retardation associated with inappropriate lack of thirst and vasopressin secretion in the presence of chronic plasma hyperosmolarity. Computed brain tomography revealed bilateral supratentorial sub-ependymal and cortical calcifications. Dissociation in the plasma vasopressin response to osmotic change was demonstrated in this patient. Treatment with a vasopressin analogue, desamino-D-arginine vasopressin (DDAVP) and forced intake of water restored plasma osmolality and serum sodium levels to normal.