Background: Hypothalamic hypodipsic syndrome is a rare condition, secondary to a defect in hypothalamic osmoreceptors that leads to impairment of water homeostasis and chronic hyperosmolality. It is frequently associated with defective osmoregulated vasopressin secretion and diabetes insipidus, and the combination results in a greater risk of hypernatremia. The principles of management consist of regulating vasopressin and water intake in relation to changes in daily body weight. Methods and Results: This case report concerns a patient who, after a postoperative cerebral infarction, developed an extensive hypothalamic syndrome including hypodipsic hypernatremia, diabetes insipidus, hyperthermia, profound short-term memory loss and initial severe anorexia, followed by hyperphagia with an inability to maintain body weight. Conclusion: The management of fluid balance proved extremely difficult in this amnesic patient, where weight alone was not sufficient to monitor water intake.