Water imbalance and consecutive electrolyte disturbances are common in the postoperative course of neurosurgical patients after pituitary surgery. Diabetes insipidus (DI) may complicate the postoperative course in as many as 30% of patients. Early and accurate diagnosis of water and electrolyte disturbances postoperatively is important for an adequate fluid and drug administration. However, identifying the causes is challenging/ ambiguous in clinical practice. Levels of antidiuretic hormone (ADH) might contribute to a straightforward diagnosis, though, its measurement is cumbersome. ADH is derived from a larger precursor peptide along with copeptin, which is a more stable peptide directly mirroring the production of ADH. Copeptin can be assayed readily in plasma. Aim: To investigate whether copeptin can accurately diagnose postoperative disturbances of water homeostasis (i.e. Diabetes insipidus and SIADH) in a cohort of patients undergoing intracranial tumor surgery.