The combination of medroxyprogesterone acetate 25 mg + estradiol cypionate 5 mg is a highly effective, monthly injectable contraceptive.  For the first time, this study presents the development and validation of a sensitive method for estradiol cypionate analysis in human plasma by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).  Aliquots (500 μL) of plasma were extracted with ethyl ether (100%) and derivatized with dansyl chloride.  Its separation was performed on a Jones Chromatography Genesis C8 column and the quantification was performed with a mass spectrometer equipped with an electrospray interface operating in negative ion mode.  The run time was 6 min and the calibration curve was linear over the range of 0.005−0.15 ng/mL. The method was applied to evaluate the pharmacokinetics of estradiol cypionate in plasma collected up to 1008 h (42 days) after a single intramuscular administration of 25 mg/mL medroxyprogesterone acetate +5 mg/mL estradiol cypionate to healthy female volunteers (n = 12).  The estradiol cypionate maximum plasma concentration (C max ) was 0.14 ± 0.08 ng/mL reached at 16.83 ± 21.07 h and the area under the plasma concentration versus time curve (AUC 0-last ) was 14.07 ± 6.32 ng.h/mL. Elimination half-life (t ½ ), apparent volume of distribution (Vd/F), apparent clearance (CL/F) and mean residence time (MRT) were 89.65 ± 76.04 h, 28038 ± 9636 L, 49.02 ± 10.62 L/h and 576.05 ± 238.32 h, respectively, showing that the estradiol cypionate release from the administration site was prolonged and there was no drug accumulation.