Patients with acute liver failure (ALF) have high mortality and frequently require liver transplantation (LT); few reliable prognostic markers are available. Levels of M30, a cleavage product of cytokeratin-18 caspase, are significantly increased in serum samples from patients with ALF who die or undergo LT. We developed a prognostic index for ALF based on level of M30 and commonly measured clinical variables (called the Acute Liver Failure Study Group [ALFSG] index) and compared its accuracy with that of the King's College criteria (KCC) and Model for End Stage Liver Disease (MELD). We also validated our model in an independent group of patients with ALF.