The study conducted by Seligman and coworkers included in the previous issue of Critical Care demonstrates that copeptin is a promising marker to predict outcome in patients with ventilator-associated pneumonia. In recent years, copeptin has emerged as a new prognostic marker in a variety of diseases, such as sepsis, community-acquired pneumonia, chronic obstructive pulmonary failure, heart failure and myocardial infarction. What is the pathophysiological basis for these findings? Copeptin together with vasopressin is co-secreted from the posterior pituitary and therefore mirrors the amount of vasopressin in the circulation. Vasopressin is a main secretagogue of the hypothalamo-pituitary-adrenal axis, thereby mirroring the individual stress level. Furthermore, vasopressin is an important hormone in salt and volume regulation. In this context, copeptin is also a diagnostic marker in patients with diabetes insipidus and in patients with disordered water states.