The progesterone receptor modulator, Ulipristal acetate (UPA) has proven to be an effective emergency contraceptive. Conflicting data has been reported that suggests different efficacy of the drug in different populations, which may be explained by the systemic exposure to the drug. A UPLC-MS/MS method was developed and validated for the accurate and sensitive measurement of UPA in human serum to address this matter. UPA was extracted from human serum using liquid-liquid extraction with a combination of hexane and dichloromethane. An analytical platform comprising reverse-phase chromatographic separation followed by mass spectrometric detection by positive electrospray ionization in multiple reaction monitoring was used for quantitation of UPA within 7min. The method was linear from 0.1 to 250ng/mL. The matrix effect was minimal and intra- and inter-assay precision and accuracy were all within the acceptable limits. UPA was found to be stable at all processing and storage conditions. The method was used to investigate the pharmacokinetics of UPA in a clinical trial designed to explore the effect of obesity on its bioavailability.