3-Hydroxyphenylacetic acid is a rutin metabolite and an antioxidant. It has a protective biological activity in human. It is a substrate of enzyme 4-hydroxyphenylacetate 3-monooxygenase [EC 1.14.13.3] in the pathway tyrosine metabolism. 3-Hydroxyphenylacetic acid is found to be associated with phenylketonuria which is an inborn error of metabolism.