Liver disease is associated with an abnormal elevation of the plasma concentrations of the aromatic amino acids phenylalanine and tyrosine. The liver is the main site of aromatic amino acid metabolism, particularly the hydroxylation of phenylalanine to tyrosine and further tyrosine degradation. In the present study, we have examined the usefulness of the L-[1-13C]phenylalanine breath test (13C-PheBT) and L-[13C]tyrosine breath test (13C-TyrBT) for the detection of hepatic damage in patients with liver cirrhosis. First, the time courses of 13CO2 excretion after the administration of L-[1-13C]phenylalanine and L-[1-13C]tyrosine were compared. The peak times (the time expressed in minutes at which 13CO2 excretion was maximal) were 20 min in both breath tests, but 13C-TyrBT gave a higher peak than 13C-PheBT. Next, the parameters of 13C-PheBT and 13C-TyrBT were compared with biochemical liver function test values. These parameters were well correlated with several liver blood test values conventionally regarded as measures of hepatocyte functional reserve. Therefore, 13C-PheBT and 13C-TyrBT may be useful to assess the degree and progression of hepatic dysfunction.