An unknown compound has been isolated in the acidic fraction of urine samples taken from several children suspected of having metabolic disorders. 2. This unknown has been characterized using a gas chromatograph/mass spectrometer/computer system. Both the high and low resolution mass spectra have been determined and a structure proposed. 3. Authentic samples were synthesized and compared to the unknown and a final proof of structure is presented. The compound, 4-hydroxycyclohexane-1-carboxylic acid, is suspected to come from a dietary source but the actual genesis will be determined in future work.