Isovalerylcarnitine is the phenotypic abnormality in isovaleric acidemia (OMIM 243500) resulting from an accumulation of isovaleric acid, which is toxic to the central nervous system. Isovaleric acidemia is an autosomal recessive inborn error of leucine metabolism caused by a deficiency of the mitochondrial enzyme isovaleryl-CoA dehydrogenase (EC 1.3.99.10) resulting in the accumulation of derivatives of isovaleryl-CoA. It was the first organic acidemia recognized in humans and can cause significant morbidity and mortality. Early diagnosis and treatment with a protein restricted diet and supplementation with carnitine and glycine are effective in promoting normal development in severely affected individuals. An alternative pathway through glycine-N-acylase (EC 2.3.1.13) allows detoxification by producing isovalerylglycine, which is excreted. Thus, isovalerylcarnitine and isovalerylglycine are the hallmarks of this disorder in plasma and urine, respectively, and are elevated regardless of a patient's metabolic condition. Moreover, isovalerylcarnitine is found to be associated with celiac disease and very long-chain acyl-CoA dehydrogenase deficiency (VLCAD), which are also inborn errors of metabolism.