BACKGROUND & AIMS The idea of acute-on-chronic liver failure (ACLF) has emerged to identify those subjects with organ failure and high mortality rates. However, the absence of a precise definition has limited the clinical application and research related to the ACLF concept. We sought to validate the ACLF definition and the CLIF-SOFA Score recently proposed by the EASL-CLIF Consortium in a cohort of patients admitted for acute decompensation (AD) of cirrhosis. METHODS In this prospective cohort study, patients were followed during their hospital stay and thirty and 90-day mortality was evaluated by phone call, in case of hospital discharge. All subjects underwent laboratory evaluation at admission. RESULTS Between December 2010 and November 2013, 192 cirrhotic patients were included. At enrollment, 46 patients (24%) met the criteria for ACLF (Grades 1, 2 and 3 in 18%, 4% and 2% respectively). The 30-day mortality was 65% in ACLF group and 12% in the remaining subjects (P < 0.001). Logistic regression analysis showed that 30-day mortality was independently associated with ascites and ACLF at admission. The Kaplan-Meier survival probability at 90-day was 92% in patients without ascites or ACLF and only 22% for patients with both ascites and ACLF. The AUROC of CLIF-SOFA in predicting 30-day mortality was 0.847 ± 0.034, with sensitivity of 64%, specificity of 90% and positive likelihood ratio of 6.61 for values ≥9. CONCLUSION In our single-centre experience the CLIF-SOFA and the EASL-CLIF Consortium definition of ACLF proved to be strong predictors of short-term mortality in cirrhotic patients admitted for AD.