The aims of this study are to investigate the feasibility and validity of the L-[1-13C] phenylalanine breath test (13C-PheBT) which has been used to measure hepatocyte functional capacity in hepatitis B virus-related liver disease patients and to propose validity parameters of the test in 12 healthy volunteer, 8 chronic hepatitis and 26 liver cirrhotic patients. 100mg/body nonradiative L-[1-13C] phenylalanine (13C-Phe) was administered orally to all subjects. Breath samples were taken before and different intervals within 360 min after administration. The 13CO2/12CO2 en-richment was assessed by isotope ratio mass spectrometer. The parameter percentage 13C excretion rate 13CERt (% 13C dose/h) all peaked within 10-30 min after oral 13C-Phe application. The parameters such as maximum value of 13C excretion rate,13CERmax (% 13C dose/h) (controls: 18.0±3.3; Child A: 11.0±3.8; Child B: 5.0±0.5; Child C: 3.6±1.2), 13C excretion rate at 30min, 13CER30 (% dose/h) (controls: 11. 9±2.1; Child A: 8.1±0.4; Child B: 6.1±0.9; Child C: 3.2±1.2), 13C cumulative excretion of first 60 min, 13Ccum60 (% 13C dose) (controls: 9.3±1.4; Child A: 6.6±0.7; Child B: 4.1±0.3; Child C: 2.6±0.9) and half time of 13C excretion rate, T1/2 (minutes) (controls: 40.4±4.4; chronic hepatitis: 53.4±4.4; Child A: 59.8±4.5; Child B: 102.0±17.3; Child C: 212.1±87.9) were effective indexes which could be employed to stage hepatocyte impairment and liver functional reserve of advanced HBV-related cirrhotic patients (i.e. healthy subjects, Child A, B, C); T1/2 was also useful for distinguishing mild HBV-related liver injure. The aims of this study are to investigate the feasibility and validity of the L- [1-13C] phenylalanine breath test (13C-PheBT) which has been used to measure hepatocyte functional capacity in hepatitis B virus-related liver disease patients and to propose validity parameters of the test in 12 healthy volunteer, 8 chronic hepatitis and 26 liver cirrhotic patients. 100 mg / body nonradiative L- [1-13C] phenylalanine (13C-Phe) was administered orally to all subjects. Breath samples were taken before and different intervals within 360 min after administration. The 13CO2 / 12CO2 en-richment was assessed by isotope ratio mass spectrometer. The parameter percentage 13C excretion rate 13CERt (% 13C dose / h) all peaked within 10-30 min after oral 13C-Phe application. (Controls: 18.0 ± 3.3; Child A: 11.0 ± 3.8; Child B: 5.0 ± 0.5; Child C: 3.6 ± 1.2), 13C excretion rate at 30 min, 13CER30 (% dose / h) (controls: 11. 9 ± 2.1; Chil (controls: 9.3 ± 1.4; Child A: 6.6 ± 0.7; Child B: 6.1 ± 0.9; Child C: 3.2 ± 1.2), 13C cumulative excretion of first 60 min, 13Ccum60 Child B: 4.1 ± 0.3; Child C: 2.6 ± 0.9) and half time of 13C excretion rate, T1 / 2 (minutes) (controls: 40.4 ± 4.4; B: 102.0 ± 17.3; Child C: 212.1 ± 87.9) were effective indexes which could be employed to stage hepatocyte impairment and liver functional reserve of advanced HBV-related cirrhotic patients (ie healthy subjects, Child A, B, C); T1 / 2 was also useful for distinguishing mild HBV-related liver injure.