Pharmacokinetic and metabolic studies of phytoestrogens of the isoflavone class have been hampered by the lack of suitable stable-isotope-labeled analogs. A method for preparation of a [(13)C]-labeled analog of daidzein is described. [2-(13)C]Daidzein was synthesized by reaction of [(13)C]diethoxydimethylaminomethane with 2,4-dihydroxybenzoin. The final product was purified to more than 99% by reverse-phase high-performance liquid chromatography and structural analysis confirmed by nuclear magnetic resonance spectroscopy and mass spectrometry. Because [2-(13)C]daidzein is analytically and metabolically stable, it is a suitable analog for use as an internal standard for quantifying daidzein in biological fluids using isotope dilution mass spectrometry. This nonradioactive tracer is also ideal for investigating the pharmacokinetics of daidzein in humans because it is biologically indistinguishable from the unlabeled form.