Abstract The urinary concentrations of free and conjugated 2-phenylethylamine were determined in phenylketonuric patients and normal subjects by solvent extraction and gas chromatography. Free 2-phenylethylamine excretion was found to be significantly elevated above normal in phenylketonuric adults and children receiving a normal or a slightly restricted intake of phenylalanine. Urinary 2-phenylethylamine was also significantly increased in phenylketonuric children receiving low phenylalanine dietary therapy. Conjugated 2-phenylethylamine excretion was found not to be increased above normal. In the light of these results, a relationship between blood phenylalanine concentration and 2-phenylethylamine excretion is proposed, and the possible role of this amine in phenylketonuria is discussed.