• We describe two sisters with chronic hypernatremia, lack of thirst, and inappropriate osmoregulated vasopressin secretion. Only one sister, who presented with microcephaly and developmental delay, showed signs of dysplasia of the midline structures (ie, septum pellucidum and corpus callosum) and a large intracranial cyst. Neither sister showed any signs of thirst, even when osmolality exceeded 337 mmol/kg. In both patients, the vasopressin secretion did not respond to either osmotic or nonosmotic stimuli or was suppressed by a water load. Plasma osmolality values returned to normal after treatment with forced hydration and a vasopressin analogue, desamino-D-arginine vasopressin. These findings indicate a severe defect in the hypothalamic osmoreceptors that control thirst and vasopressin secretion. To our knowledge, this is the first report of such a disorder in two sisters.