The syndrome of polyuria and polydipsia is practically identical in three very different disorders (central diabetes insipidus, nephrogenic diabetes insipidus, primary polydipsia). In congenital nephrogenic diabetes insipidus both the thirst mechanism and the production of the antidiuretic hormone are intact, but the hormone is ineffective in the kidney. The acquired form of the disease is caused most frequently by tubulointerstitial and urinary tract obstructive disorders as well as among the several drugs, lithium. In the differential-diagnosis the results of determinations related to the nomograms of normal interrelationships between osmolality and vasopressin in urine and plasma are used, besides the classical Carter-Robbins and indirect dehydration (Miller) tests. It has been postulated recently that in many cases the resistancy toward vasopressin is not absolute and these partially vasopressin sensitive patients can be treated successfully by thiazide, and potassium sparing compounds, antiprostaglandin pain killers (non-steroid antiinflammatory drugs) and first of all dDAVP as well as combinations of these preparations.