During the analysis of urine for organic acids for suspected metabolic disorders by solvent extraction, derivatisation and capillary gas chromatography, unaccountably large lactic acid peaks were observed in some samples containing large amounts of acetoacetic acid. Electron impact mass spectrometry showed that this was due to two unknown compounds coeluting with lactic acid. These were found to be two trimethylsilyl derivatives of 3-methylisoxazol-5-one, produced from acetoacetic acid during oximation with hydroxylamine hydrochloride, by a cyclisation reaction. Awareness of the formation of this previously unreported artefact is important to laboratories employing a similar profiling procedure.