A sensitive and accurate stable isotope dilution GC/MS assay was developed and validated for the quantification of succinylacetone (SA) in plasma and amniotic fluid (AF). SA is pathognonomic for tyrosinemia type I, a genetic disorder caused by a reduced activity of fumarylacetoacetate hydrolase (FAH). In untreated patients, SA can easily be measured in plasma and urine because the expected concentrations are in the micromol/L range. Due to a founder effect, the province of Quebec has an unusually high prevalence of tyrosinemia type I, hence, the quantification of SA in AF or plasma of treated patients in the nmol/L range becomes very useful. The method utilizes 13C5-SA as an internal standard and a three-step sample treatment consisting of oximation, solvent extraction and TMCS derivatization. The assay was validated by recording the ion intensities of m/z 620 for SA and m/z 625 for ISTD in order to demonstrate the precision of measurements, the linearity of the method, limit of quantification and detection (LOQ and LOD), specificity, accuracy, as well as metabolite stability. Values for the intra-day assays ranged from 0.2 to 3.2% while values for the inter-day assays ranged from 1.9 to 5.6% confirming that the method has good precision. A calibration plot using SA detected by GC/MS gave excellent linearity with a correlation coefficient of 0.999 over the injected concentration range of 5-2000 nmol/L. LOQ and LOD were 3 and 1 nmol/L, respectively. The usefulness of this method was demonstrated by SA quantification in an AF sample of an affected fetus and in plasma of patients treated with NTBC. The results demonstrate that this novel GC/MS method may be a valuable tool for metabolic evaluation and clinical use.