To estimate the contraceptive failure rates of the etonogestrel subdermal contraceptive implant in overweight and obese women and compare failure rates with women of normal weight and women using intrauterine devices (IUDs). The Contraceptive CHOICE Project is a large prospective cohort study designed to promote the use of long-acting reversible contraceptive methods to reduce unintended pregnancies in the St Louis region. Participants are provided reversible contraception of their choice at no cost. We collected baseline height and weight of each participant. During each survey, participants were asked about missed menses and possible pregnancies. Any participant who suspected a pregnancy was asked to come in for urine pregnancy testing. Analysis includes the first 8,445 participants enrolled in CHOICE of which 1,168 chose the implant and 4,200 chose the IUD. Student's t test, χ test, and Kaplan-Meier survival curves were used to perform statistical analyses to estimate failure rates in overweight and obese women using the implant and IUDs. Of the women choosing the implant, 28% were overweight and 35% were obese. Of the women who chose an IUD, 27% were overweight and 35% were obese. The 3-year cumulative failure rates for implant and IUD users were less than one per 100 women-years and did not vary by body mass index. We found no decrease in the effectiveness of the implant in overweight or obese women. The implant may be offered as a first-line contraceptive method to any woman seeking a reversible and reliable birth control method. II.