Etonogestrel (ENG) is a progestin used in the contraceptive vaginal ring NuvaRing and the subdermal implant Implanon. A sensitive method for measuring ENG is useful for further investigating the progestin's pharmacokinetics with these alternative contraceptive formulations and generating important information about possible continued efficacy or potential failure to remove the subdermal implant.Standards and serum samples were spiked with D8-progesterone (internal standard) and subsequently extracted with dichloromethane, dried, and reconstituted in 25% methanol with formic acid. ENG was analyzed by positive electrospray ionization in multiple reaction monitoring mode with a run time of 5.5 minutes using a C18 BEH column. The mobile phase was a gradient of water:acetonitrile, with 0.1% formic acid. The method was applied successfully to study the pharmacokinetics of ENG during vaginal ring use. The method was also used in routine patient care to assess ENG levels.The method is linear from 50 to 2000 pg/mL. The limits of detection and quantification are 25 and 50 pg/mL, respectively. There was no observed ionization suppression within the linear range of the assay, and the average recovery was 87%. Serum ENG levels of n = 3 subjects were all within the linear range of the assay for a total study period of 42 days after insertion of the ring. Of n = 20 patients with nonpalpable subdermal implants, n = 13 had ENG levels >25 pg/mL, whereas n = 7 had levels <25 pg/mL.We developed a rapid, sensitive, and robust ultra performance liquid chromatography-tandem mass spectometry (UPLC-MS/MS) method for the quantification of ENG in serum that is useful to study the progestin's pharmacokinetics and inform physicians about successful implantation or potential failure to remove a subdermal device.