The differential diagnosis of the polyuria-polydipsia syndrome has been notoriously cumbersome, often involving some modified form of the indirect water deprivation test. Furthermore, published vasopressin ranges for the various forms of diabetes insipidus overlap; and reliable assays, tedious and time consuming as they are, never found their way into clinical routine. Now, the introduction of a routine-proof assay for copeptin, the biochemically stable C-terminal glycopeptide segment of the vasopressin prohormone, paved the way out of this stalemate.