The aim of this study is to evaluate the feasibility and safety of random-start controlled ovarian hyperstimulation (COH) for emergency fertility preservation, regardless of the phase of the menstrual cycle. A self-controlled pilot clinical trial (NCT01385332) was performed in an acute-care teaching hospital and in two private reproductive centers in Barcelona, Spain. Eleven egg donors participated in the study. Two random-start gonadotropin-releasing hormone (GnRH) antagonist protocols were assessed in which ganirelix was initiated on either day 10 (protocol B) or on day 20 (protocol C) of the menstrual cycle and was continued until estradiol levels were below 60 pg/dL. These protocols were compared with a standard protocol (protocol A). The main outcome of interest was the number of metaphase 2 oocytes retrieved. Results from this study show that the number of mature oocytes retrieved was comparable across the different protocols (14.3±4.6 in the standard protocol versus 13.0±9.1 and 13.2±5.2 in protocols B and C, respectively; values expressed as mean ± standard deviation). The mean number of days needed for a GnRH antagonist to lower estradiol levels, as well as the ongoing pregnancy rates, were also similar when protocols B (stimulation in follicular phase) and C (stimulation on luteal phase) were compared with protocol A (standard stimulation). GnRH antagonists can be effectively used for random-start controlled ovarian hyperstimulation with an ovarian response similar to that of standard protocols, and the antagonists appear suitable for emergency fertility preservation in cancer patients.