N-carbamoyl-beta-D-glucopyranosylamine (NCG) is a compound, which can chemically be synthesized by the condensation of glucose and urea by heating under acidic condition. In this study, we isolated and identified NCG and its anomer from human serum. This is the first study showing the occurrence and isolation of NCG from a natural source. The NCG level in human serum was estimated to be 71+/-33 microM using a glucose-3-dehydrogenase-based assay. Because NCG is not commercially used in foods or drugs, we conclude that the NCG isolated from human serum is synthesized from blood glucose and urea in vivo.