Duration of efficacy and prevalence of side-effects associated with GonaCon Immunocontraceptive Vaccine (GonaCon) in free-ranging female elk (Cervus elaphus) are unknown. In January 2008, we captured 120 mature female elk in Rocky Mountain National Park (CO, USA), determined pregnancy status, and randomly assigned them to treated (n = 60; 1.5 mL of GonaCon) or control (n = 60; 1.5 mL of saline) groups. During the following 3 winters we recaptured, collected blood for antibody concentrations, and euthanized 10–20 elk in each group. At necropsy, we determined pregnancy and collected tissues from organs associated with the hypothalamic–pituitary–gonadal axis. We relocated injection sites, collected muscle tissue, and performed bacterial culture when inflammation was present. Proportion of pregnant elk among control females ranged from 0.75 to 0.90. Proportion pregnant after treatment with GonaCon was 0.00 (95% CI = 0.0–0.22) in year 1, 0.31 (CI = 0.09–0.61) in year 2, and 0.65 (CI = 0.41–0.85) in year 3. Antibody concentrations were higher in non-pregnant than pregnant treated females. We found no antemortem evidence of lameness or swelling at the injection site; however, at necropsy all treated females had pyogranulomatous inflammation at the injection site. We observed no consistent changes within the hypothalamic–pituitary–gonadal axis. We conclude that GonaCon is effective at reducing pregnancy for 1–2 years post-vaccination and is strongly associated with sterile inflammation at the site of injection. Similar to other species, the vaccine is less effective in elk under free-ranging conditions than those in a captive environment. © 2014 The Wildlife Society.