13C-phenylalanine (PheBT) and 13C-galactose breath tests (GBT) explore non invasively the hepatic functional mass by measuring two enzymatic activities localized into the cytosol of liver cells: the phenylalanine hydroxylase (which converts phenylalanine into tyrosine) and the galactose kinase (which catalyzes the ATP-dependent phosphorylation of galactose to galactose 1-phosphate). Both BTs are safe and accurate in predicting the severity of liver cirrhosis showing a good correlation with the Child-Pugh score. PheBT is also used in predicting postoperative complications and monitoring liver regeneration in patients undergoing partial hepatectomy. GBT has been also used to assess liver fibrosis in patients with chronic hepatitis C. PheBT and GBT could be used in the diagnosis of two inborn errors of metabolism, phenylketonuria and galactosemia, respectively. Both BTs are not affected by enzymatic induction due to drugs which may interfere with the results of the classic "microsomial" BTs (such as the aminopyrine or caffeine BTs).