The finding of a polyuropolydispsic syndrome should prompt a complete biological investigation of its etiology. If polyuria can be a minor sign as in psychiatric disorder, it can also be the first manifestation of diabetes mellitus but also central diabetes insipidus, the latter linked to cerebral tumors, metastases in the hypothalamus, granulomatous disease, but also nephrogenic diabetes insipidus such as chronic renal disease or autoimmune disease. Intracellular dehydration is the major risk in case of a polyuropolydipsic syndrome. Prognosis depends on the capacity to maintain water balance through an intact thirst mechanism. After a brief review of the majority of causes of diabetes insipidus, we propose a diagnosis algorithm to easily make the diagnosis.