Normal water balance with strict maintenance of plasma osmolality depends on appropriate water conservation (controlled by ADH release and action) and additional water intake if required (triggered by the awareness of thirst). Central nervous system pathology (including trauma) commonly involves the hypothalamus and pituitary stalk, leading to impaired osmoreceptor function or diminished ADH production or release, resulting in diabetes insipidus (with potentially life-threatening abnormalities in fluid and electrolyte status). Assessment of the relationships between plasma and urine osmolality and plasma ADH levels will usually lead to an accurate diagnosis. Central diabetes insipidus is effectively treated with replacement of free water deficits and exogenous ADH analogues.