Abstract Study question Does estrogen pretreatment in antagonist ICSI cycles improves the outcome? Summary answer Pretreatment with estrogen in antagonist cycles reduced the days of stimulation, the number of hMG ampoules and enhanced the implantation rate. What is known already It has been previously reported that, programming of antagonist cycles with E2 only, started in the luteal phase and continued beyond the menses till the first day of stimulation, yielded similar results as the long agonist protocol. E2 pre-treatment in the luteal phase has an inhibiting role on follicular growth through its negative feedback on the FSH increase during the luteal follicular transition phase. This effect stops as soon as E2 administration is terminated. Study design, size, duration This was a prospective randomized controlled trial performed in a private IVF centers from June 2020 to February 2021. One hundred and fourteen infertile couples undergoing intracytoplasmic sperm injection embryo transfer cycles (ICSI-ET) were included. Participants/materials, setting, methods Group A includes 57 patients were pretreated with a dose of 4 mg estradiol valerate (progynova 2 mg; Bayer) once daily starting 10days before expected menses. Ovarian stimulation was started one day after the last estradiol dose, in a dose ranging from 225 to 375 IU of hMG. While in group B , 57 patients received stimulation in a dose of 225 to 375 IU of hMG starting on cycle day 2 without estradiol pretreatment. Main results and the role of chance The daily dose of hMG was significantly lower in The study group (P = 0.048), the stimulation days were significantly higher in the control group (P = 0.049), the total number of hMG ampoules was significantly higher in the control group (P = 0.041), and the implantation rate was higher in the study group (P = 0.045) , also the pregnancy rate (positive pregnancy test) was higher in the study group (p = 0.044) but there was no significant statistical difference detected as regards the clinical pregnancy rate between both groups (P = 0.180) , the serum estradiol on day of hCG ( p = 0.055), serum progesterone level on hCG day (P = 0.71) and the endometrial thickness (p = 0.780) . Also there is no significant statistical difference between the two groups as regarding number of follicles on day of hCG. (P = 0.803), the number of mature oocytes (P = 0.703) and no significant difference in the number of good quality embryos (p = 0.532) and the ratio of mature oocytes to number of follicles on day of hCG (P = 0.835), with no significant statistical difference between both groups. Limitations, reasons for caution The limitation of this study is the small number of patients as we included only 114 patients. Wider implications of the findings Luteal estradiol in antagonist cycles improved the outcomes of ICSI procedure by reducing the number of gonadotropin ampoules used for COH and reduced duration of stimulation, also the implantation rate is improved by luteal estradiol administration. Trial registration number NCT05197374