BACKGROUND & AIMS: Research on vasopressin (AVP) in cirrhosis and its role in the assessment of prognosis has been hindered by the difficulty of measuring AVP levels accurately. Copeptin, a 39-aminoacid glycopeptide, is released from the neurohypophysis together with AVP. Copeptin could have a role as biomarker of prognosis in cirrhosis as it may reflect circulatory dysfunction. The aim of this study is to investigate the role of copeptin as biomarker of disease progression and prognosis in cirrhosis. METHODS: This prospective study is divided in 2 study protocols including 321 consecutive patients. Plasma copeptin levels were measured in all patients at study inclusion. Protocol 1: to investigate the relationship of copeptin with kidney and circulatory function (56 patients). Protocol 2: to investigate the relationship between copeptin and prognosis, as assessed by the development of complications of cirrhosis or mortality at 3months (265 patients admitted to hospital for complications of cirrhosis). RESULTS: Patients with decompensated cirrhosis showed significantly higher plasma copeptin levels compared to those of patients with compensated cirrhosis. Copeptin levels had a significant positive correlation with model for end-satge liver disease (MELD) score, AVP, endogenous vasoconstrictor systems, and kidney function parameters. Patients developing complications of cirrhosis or mortality had significantly higher plasma copeptin levels compared to those of the remaining patients. Plasma copeptin levels were an independent predictive factor of both the development of complications and mortality at 3months. This was confirmed in a validation series of 120 patients. CONCLUSIONS: Copeptin is a novel biomarker of disease progression and prognosis in cirrhosis. LAY SUMMARY: Copeptin is a fragment of the vasopressin precursor, a hormone that is known to be increased in patients with cirrhosis and that plays a role in the development of complications of the disease. Vasopressin is difficult to measure, but copeptin is a more stable molecule and is easier to measure in blood. Solà and Kerbert and colleagues have shown in a series of 361 patients that copeptin is markedly increased in patients with cirrhosis who develop complications during the following 3months, compared to those patients who do not develop complications. Moreover, copeptin correlates with prognosis.