Endometrial receptivity is a primary concern for embryo implantation success in fertility treatments. The present study was a retrospective analysis of 4070 cycles with donor oocytes and hormone-replacement therapy. Endometrial thickness was assessed once with transvaginal ultrasound. Patients were allowed to continue when endometrial thickness was ?5mm and had triple line morphology. Pregnancy rates, the number of gestational sacs and miscarriage rates were analysed in relation to endometrium status. Regression models were used to analyse associations, taking the day of embryo transfer into account. All patient parameters were homogeneous. Mean endometrial thickness was 7.24±1.66mm, the mean number of embryos transferred was 2.04±0.43, the pregnancy rate was 48.06% and sacs were present in 42.3% of cycles. There were no significant differences in pregnancy rates, number of gestational sacs and miscarriage rates for different endometrial thickness measurements. The present study is, to our knowledge, the largest study evaluating the role of endometrial thickness in oocyte donation cycles. Endometrial thickness >5mm is a reasonable parameter for determining treatment success, and once it is observed in a single ultrasonographic evaluation there is no need for subsequent monitoring and embryo transfer can be scheduled over the following 1-16 days, because the results are not compromised. This may lead to a significant reduction in time and cost in fertility clinics.