OBJECTIVE: To evaluate the effect of the addition of estradiol to luteal progesterone supplementation in GnRH antagonist cycles for infertile patients undergoing IVF/ICSI. METHODS: One hundred and ten infertile patients, aged 28 to 39 years, were recruited for this prospective randomized study. They were randomly assigned to receive vaginal progesterone gel (Crinone) along with 4 mg estradiol valerate (group 1, n=55) or only Crinone (group 2, n=55) for luteal support. A GnRH antagonist multiple dose protocol using recombinant human FSH was used for controlled ovarian stimulation (COS) in all of the subjects. The COS results and pregnancy outcomes of the two groups were compared. RESULTS: Group 1 and 2 were comparable with respect to the patient characteristics. The COS and IVF results were also comparable between the two groups. There were no differences in the clinical pregnancy rate (PR) and multiple PR between the two groups. However, the embryo implantation rate were significantly higher in group 1 than that in group 2 (22.2% vs. 13.3%, p=0.035). The incidence of luteal vaginal bleeding (LVB) was significantly lower in group 1 (7.4% vs. 27.8%, p=0.010). CONCLUSION: The addition of estradiol to luteal progesterone supplementation in GnRH antagonist cycles reduces the incidence of LVB and increases the embryo implantation rate in infertile patients undergoing IVF/ICSI.