Objective: To evaluate the prognostic value of the endometrial thickness assessment on the day of embryo transfer (ET) on pregnancy outcome. The main outcome measures are pregnancy rates and live births. Methods: Fifty-one patients, <= 37 years of age, undergoing their first cycle of ICSI, were prospectively studied from May 2004 to May 2005. They were prescribed to the same protocol for ovarian stimulation with recombinant-follicular stimulating hormone (r-FSH) and GnRH antagonist. During ET, the endometrial thickness was evaluated by abdominal ultrasonography (US). Results: There were no statistically significant differences in age, duration of stimulation, total dose of follicular stimulating hormone (FSH), number of metaphase II (MII) oocytes retrieved, and number of good quality embryos transferred among pregnant (39%) and nonpregnant (61%) patients. Also, no statistically significant difference in endometrial thickness on the day of ET was observed. Similar results were observed for the comparison of patients who had a live birth (25%) with the rest (75%) showing no statistically significant difference in age, duration of stimulation, total dose of FSH, number of MII oocytes retrieved, and number of good quality embryos transferred. Furthermore, no statistically significant difference in endometrial thickness on the day of ET was observed. Conclusions: The prognostic question of uterine receptivity has not yet been solved through the use of ultrasound technology. Our results did not show statistically significant relationship between the endometrial thickness on the day of ET and pregnancy rates or live births in patients using GnRH antagonists.