BACKGROUND AND AIMS. Acute-on-chronic liver failure has been recognized as a sudden deterioration of cirrhosis, with a high short-term mortality. Prognostic scores are used to assess liver dysfunction. However, there is not enough information on a score to predict short term mortality in those patients. We aimed to investigate the prognostic value of bilirubin concentration in predicting the 1-week outcome of patients with acute-on-chronic liver failure. MATERIAL AND METHODS. We performed a retrospective analysis with a cohort of 65 patients (33 women/32 men), age average of 64 years, diagnosed with acute-on-chronic liver failure with at least 1 week follow-up. Demographics, clinical and biochemical variables were analyzed. Most patients died (59 %) within 1 week of follow-up. RESULTS. In univariate logistic regression analysis, admission to the intensive care unit, use of vasoactive drugs, need for parenteral nutrition, and levels of conjugated, unconjugated, and total bilirubin at the time of hospital admission were significantly associated with 1-week mortality; in a multivariate logistic regression, conjugated (p = 0.01), unconjugated (p =0.01), and total bilirubin (p = 0.009) were independently associated with 1-week mortality. In ROC curve analysis, conjugated (0.751, p < 0.05) and total bilirubin (0.746, p < 0.05) levels were significantly the best short-term mortality predictors. CONCLUSIONS. High levels of bilirubin are able to predict short-term mortality in these patients. Also, we suggest that bilirubin can be used as a biochemical marker to improve triage of patients with acute-on-chronic liver failure especially with emerging interventions such as extracorporeal liver assist devices and possibly improved early phase pharmacological therapies.