A sensitive, selective, and quantitative method for the simultaneous determination of gemcitabine and 2,2-difluoro-2-deoxyuridine (dFdU) has been developed and validated in human plasma in the presence of tetrahydrouridine, a cytidine deaminase inhibitor. The method employs derivatization of gemcitabine and dFdU with dansyl chloride to improve the chromatographic retention and separation. The derivatization was performed in plasma without prior sample clean-up, followed by extraction of the dansyl-derivatives using methyl tertiary-butyl ether (MTBE). Ultra performance liquid chromatography (UPLC) technology on a BEH C18 stationary phase column with 1.7 microm particle size was used for chromatographic separation coupled to tandem mass spectrometry. The method was validated over the concentration ranges of 20-5000 and 100-25,000 ng/mL for gemcitabine and dFdU, respectively. The results from assay validation show that the method is rugged, precise, accurate, and well-suited to support pharmacokinetic studies. In addition, the relatively small sample volume (50 microL) and a run time of 1.5 min facilitate automation and allow for high-throughput analysis.