A number of previously unrecognized abnormal metabolites have been identified and quantitated in the urine of a patient with an inherited deficiency of propionyl-CoA carboxylase. These included the isoleucine metabolites 2-methyl-3-hydroxybutyric acid and 2-methylacetoacetic acid. These isomers 3-hydroxyvaleric acid and 3-oxovaleric acid were found, which may be products of the condensation of propionyl-CoA with acetyl-CoA catalyzed by 3-oxoacyl-CoA thiolases. Following a load of isoleucine, 2-methylbutyrylglycine was identified. This metabolite has not previously been observed in man.