(S)-2-Hydroxy-3-phenylpropanoic acid (L-(−)-3-Phenyllactic acid) is a chiral aromatic compound involved in phenylalanine metabolism. It is likely produced from phenylpyruvate via the action of lactate dehydrogenase. The D-form of this organic acid is typically derived from bacterial sources while the L-form is almost certainly endogenous. Levels of phenyllactate are normally very low in blood or urine. High levels of PLA in the urine or blood are often indicative of phenylketonuria (PKU) and hyperphenylalaninemia (HPA). PKU is due to lack of the enzyme phenylalanine hydroxylase (PAH) so that phenylalanine is converted not to tyrosine but to phenylpyruvic acid (a precursor of phenylacetate).