Progesterone elevation occurring in the late phases of controlled ovarian stimulation (COS) has been reported for over 25 years. Yet doubts remain regarding the mechanisms at play in this phenomenon and its net consequences on assisted reproductive technology outcome, which is known to occur in poor and good assisted reproductive technology responders. The pathophysiology of end-COS progesterone elevation encountered in gonadotropin-suppressed cycles is different from that prevailing at the time of, and just after, ovulation. The different divergence in practical consequences of end-COS progesterone elevation led to review the progesterone assays developed for measuring progesterone in the luteal phase of the menstrual cycle, but commonly used for measuring all forms of progesterone elevation.