Objective: To evaluate the efficacy of estradiol supplementation starting on the day of human chorionic gonadotrophin (hCG) in patients with thin endometrium in intracytoplasmic sperm injection (ICSI) cycles. Methods: A total of 117 consecutive patients with the endometrial thickness on the hCG day <= 8 mm were rewieved. Estradiol supplementation was given in 57 patients and the remaining 60 patients were accepted as control group. Estradiol supplemented (ES) group received estradiol hemihydrate 4 mg/day started on the day of hCG. Luteal phase was supported using the vaginal progesterone gel in both groups. Clinical pregnancy rate, implantation rate, miscarriage rate, endometrial thickness on the day of oocyte pick-up and on the day of embryo transferred were accepted as main outcome measures. Results: There were no statistical differences in terms of clinical pregnancy rate (28.1% vs. 23.3%), implantation rate (16% vs. 10.4%), miscarriage rate (21% vs. 31.6%), endometrial thickness on the oocyte pick-up day (8.5 +/- 1.8 vs. 8.4 +/- 1.4, mm) and embryo transferred day (9.6 +/- 2.9 vs. 10.3 +/- 2.4, mm) in the ES group vs. control group. Conclusion: Estradiol supplementation starting on the hCG day for the patients with thin endometrium does not provide any benefit on the pregnancy outcome in ICSI cycles.