We describe an isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of succinylacetone (SA) in urine for the diagnosis of hepatorenal tyrosinemia (HT1). The method used 15N-labeled 5(3)-methyl-3(5)-isoxazole propionic acid as internal standard. Urine samples were oximated with hydroxylamine hydrochloride at 80 degrees C, extracted by solvent-solvent extraction, and followed by derivatization of the butyl ester. The butylated isoxazole derivatives of SA and its internal standard were detected and quantified using positive ion electrospray LC-MS/MS with selected reaction monitoring. The turnaround time between injections was 10 min. Calibration curves were linear over the range of 0.0633-63.3 micromol/L. The intra- and interday assay variations were less than 7%. Mean recoveries of SA at three different concentrations ranged from 96 to 109%. During the course of this study, we identified 12 new patients with HT1 and applied this method to follow up the treatment of 4 of these patients as well as previously diagnosed HT1 patients.