Background: Liver failure is associated with low concentrations of branched chain amino acids and high concentrations of most other amino acids. In this study the effect of treatment with the Molecular Adsorbents Recirculating System (MARS) on arterial amino acid levels and cerebral amino acid metabolism was examined in patients with severe hepatic encephalopathy. Methods: The study included seven patients with hepatic encephalopathy from fulminant hepatic failure (FHF) and five patients with hepatic encephalopathy from acute on chronic liver failure (AoCLF). Cerebral blood flow and cerebral arteriovenous differences in amino acids were measured before and after 6 h of treatment with MARS. Results: During MARS treatment, the total arterial amino acid concentration decreased by 20%from 8.92 ±7.79 mmol/L to 7.16 ±5.64 mmol/L (P < 0.05). The concentration decreased in all amino acids with the exception of the branched chain amino acids. Fischer’s ratio of branched chain to aromatic amino acids increased from 0.73 ±0.47 to 0.91 ±0.54 (P < 0.05). A net cerebral efflux of amino acids in patients with FHF (8.94 ±8.34 μmol;/100 g/min) as well as AoCLF (7.35 ±24.97 μmol/100 g/min) was not affected by the MARS treatment. MARS had no effect on the cerebral metabolic rate of any single amino acid in either group. Conclusions: MARS treatment tends to normalize the arterial amino acid concentrations in patients with hepatic encephalopathy. Even though the overall reduction in plasma amino acids and improvement in amino acid dysbalance may well be beneficial, it was not accompanied by any immediate improvement in cerebral amino acid metabolism in patients with FHF or AoCLF. Background: Liver failure is associated with low concentrations of branched chain amino acids and high concentrations of most other amino acids. In this study the effect of treatment with the Molecular Adsorbents Recirculating System (MARS) on arterial amino acid levels and cerebral amino acid metabolism was examined in patients with severe hepatic encephalopathy. Methods: The study included seven patients with hepatic encephalopathy from fulminant hepatic failure (FHF) and five patients with hepatic encephalopathy from acute on chronic liver failure (AoCLF). Cerebral blood flow and cerebral arteriovenous differences in amino acids were measured before and after 6 h of treatment with MARS. Results: During MARS treatment, the total arterial amino acid concentration decreased by 8.9% ± 7.79 mmol / L to 7.16 ± 5.64 mmol / L (P <0.05). concentration decreased in all amino acids with the exception of the branched chain amino acids. Fischer’s ratio of branched chain to aromatic am Ino acids increased from 0.73 ± 0.47 to 0.91 ± 0.54 (P <0.05). A net cerebral efflux of amino acids in patients with FHF (8.94 ± 8.34 μmol / 100 g / min) as well as AoCLF (7.35 ± 24.97 μmol / MARS had no effect on the cerebral metabolic rate of any single amino acid in either group. Conclusions: MARS treatment tends to normalize the arterial amino acid concentrations in patients with hepatic encephalopathy. Even though the overall reduction in plasma amino acids and improvement in amino acid dysbalance may well be beneficial, it was not accompanied by any immediate improvement in cerebral amino acid metabolism in patients with FHF or AoCLF.