OBJECTIVE Ovarian follicle and cyst formation have been recognized as an advance phenomenon associated with GnRH agonist administration. With the use of GnRH antagonists, pituitary suppression is immediate and no flare effect and follicle growth are expected. We describe two patients who developed a dominant follicle and presumably ovulated in response to hCG triggering under continuous sole administration of a GnRH antagonist. METHODS Case report. METHODS An IVF unit at a university hospital. METHODS Two young healthy female patients undergoing IVF because of male-factor infertility. METHODS Continuous daily administration of a GnRH antagonist from menstruation with the aim of achieving ovarian suppression. METHODS Endocrine and ultrasound characteristics of follicular growth. RESULTS Both patients developed a dominant follicle under sole administration of a GnRH antagonist, accompanied by a gradual rise in serum estradiol and endometrial thickness which culminated in a spontaneous LH surge. Ovulation was triggered by hCG and mid-luteal progesterone levels were suggestive of ovulation. CONCLUSIONS We describe for the first time the development of a dominant follicle and presumable ovulation under continuous administration of a GnRH antagonist. Serum gonadotropin concentrations indicate that the pituitary failed to suppress in both patients. The exact mechanism of this phenomenon remains to be elucidated.