An unknown urinary amino acid, present in small amounts in many children with various diseases, has been isolated and identified as N-Epsilon-(carboxymethyl)lysine. The identity of this compound was confirmed by synthesis. Its chromatographical characteristics are described. The compound also occurred in the urine of prematurely born infants. Even in healthy adults trace amounts could be detected. In a few patients strikingly higher excretions were observed, up to 67 mg/g creatinine, as determined by cation-exchange column chromatography. No clear correlation with a distinct clinical picture could be established. Speculations are given about the origin of the compound.