PURPOSE: A thin endometrium is one of the most difficult problems encountered in assisted reproduction every day practice. Whether a daily dose of 150 IU HCG for 7 days concomitant with estrogen administration in estrogen replacement cycles can increase the endometrial thickness and improve pregnancy outcome, was the objective of the current study.METHODS: Seventeen infertile patients with successive implantation failures and resisting thin endometrium, being recipients of fresh donor or frozen embryos were recruited. This was a prospective cohort, proof of concept study, NCT01768247. On day-8 or 9 of the estrogen administration, and continuing 8 mg estrogen per day, subcutaneous injections of 150 IU HCG were initiated daily for 7 days. After a week on HCG priming, (day-14 or 15) endometrial thickness was controlled with ultrasound, and progesterone was initiated.RESULTS: Mean endometrial thickness was increased from 5.2 mm to 6 mm (p = 0.008). 35.3 % of the patients had more than 20 % improvement of their endometrial thickness after HCG priming. 17 % achieved an endometrial thickness more than 7 mm, and 29.4 % did not improve their thickness at all. Interestingly, from the later two became pregnant. Overall, 41 % of them (7/17) finally delivered.CONCLUSIONS: One hundred fifty IU HCG endometrial priming for 7 days in the proliferative phase of estrogen substituted cycles for frozen embryos is highly promising, as not only the thickness of the endometrium improves but also eventually the receptivity appears normalized.