Acute liver failure (ALF) is a medical emergency. Molecular adsorbent recirculating system (MARS), an artificial liver support system, can partly compensate for the detoxifying function of the liver by removing toxins from blood. To analyze the efficacy of MARS treatment, the outcomes of 113 ALF patients, treated with MARS between 2001 and 2007, were compared with a historical control group of 46 ALF patients treated without MARS between 1995 and 2001. Overall survival of transplanted patients was 94% in the MARS group and 77% in the control group (P=0.06). Without transplantation, survival was 66% and 40% (P=0.03), respectively. However, the etiological distribution of ALF differed significantly between the groups. In ALF patients with unknown etiology, groups were comparable at baseline; 91% and 69% of transplanted patients survived the MARS and control groups and the native liver recovered in 20% and 8% of the patients, respectively. Of the originally nonencephalopathic patients of unknown etiology, 36% underwent liver transplantation in the MARS group compared to 100% in the control group. Interpretation of the results was difficult in toxic etiology patients on account of differing baseline statuses. MARS treatment might partly explain the trend toward increased survival of ALF patients with unknown etiology.