Gestational diabetes insipidus is a rare disorder characterized by polyuria and polydipsia due to the inability of the kidneys to concentrate urine. We report two cases of transient gestational diabetes insipidus in which patients responded to intranasal DDAVP (1-deamino-8-D-arginine vasopressin) with greater than 50% increase in urine osmolality and marked reduction in urine output. Intranasal DDAVP was discontinued after their discharge and both patients maintained normal urine output and appropriate urine osmolality. In determining whether diabetes insipidus is present in a patient who is polyuric and hypernatremic, a urine osmolality below that of the plasma suggests the presence of diabetes insipidus. Understanding of the pathophysiology may soon lead to improved methods of prevention, diagnosis and treatment.