An isotope dilution liquid chromatography/tandem mass spectrometry is proposed as a reference method to determine the level of tyrosine in human serum. The advantages of this method include simple sample preparation without derivatization, the selective detection of compounds of interest in complex matrices, and the use of an isotopically labeled analogue as an internal standard. Tyrosine and its isotopically labeled analogue were monitored at a transfer m/z of 182.1/136.2 and 188.1/142.2, corresponding to [M+H]+/[M+H-HCOOH]+ in a multiple reaction monitoring mode. The expanded uncertainty for the measurement of tyrosine in the serum was approximately 0.95% within a 95% confidence level. For the verification of this method, a standard reference material with a certified value was analyzed. The analyzed result was in good agreement with the certified value. The isotope dilution liquid chromatography/tandem mass spectrometry result of the human serum was also compared with results obtained from clinical laboratories, and showed inconsistent results. These inconsistent results suggest that standards certified by the proposed reference method are required in order to improve measurement reliability in clinical fields.