Analysis: We will study 5 groups of patients: A) Patients with complete central diabetes insipidus, B) Patients with partial central diabetes insipidus, C) Patients with complete nephrogenic diabetes insipidus, D) Patients with partial nephrogenic diabetes insipidus and E) Patients with psychogenic diabetes insipidus. All groups will consist of 10 patients based on the following assumptions: Based on pilot studies we assume that patients in group A) will have copeptin values of 2.5 ± 1.0; Group B) 3.0 ± 1.0, Group C) 15.0 ± 5; Group D) 6 ± 2.0 and Group E) 4.0 ± 1.0 pmol/L. This results in a power of 90% to detect a difference in copeptin levels of 0.8pmol/L between the closest two groups, i.e. patients with partial central Diabetes insipidus and patients with psychogenic Diabetes insipidus.