The physiologic basis and clinical applications of the use of GnRHa, rather than hCG, to induce the final stage of oocyte maturation and ovulation in gonadotropin-treated cycles were reviewed. A single mid-cycle dose of GnRHa is able to trigger a preovulatory LH/FSH surge, leading to oocyte maturation and pregnancy in women undergoing ovarian stimulation for IVF/ET or induction of ovulation in vivo. The limited information currently available suggests there are similar pregnancy rates in patients treated with either GnRHa or hCG. The potential clinical advantages of GnRHa over hCG in gonadotropin-treated cycles include 1) the ability to titrate the amplitude and duration of the LH surge, 2) better control of luteal steroid hormone levels, 3) a higher implantation rate, 4) a lower rate of multiple pregnancy, and 5) a reduced risk of OHS. To date, the GnRHa regimen has been effective in preventing OHS in patients at high risk for having this complication.