Purpose: To study the correlation between endometrial thickness and IVF outcome and factors affecting this relation. Methods: Transvaginal ultrasound evaluation of endometrial thickness on hCG administration day in 112 IVF-ET cycles and comparison to indices of ovarian stimulation. Outcome was considered positive when fetal sac and fetal heart pulse were present at ultrasound. GnRH-agonist and antagonist protocols were also compared. Statistical analysis was performed by the SPSS system, chi-square and t-test. Results: 38 cycles displayed clinical pregnancy. In cases of higher endometrial thickness, pregnancy rates, mean serum estradiol levels, oocyte and mature oocyte numbers as well as mean large follicle numbers were higher, while the mean age was lower. Conclusions: In 38 cycles resulting in pregnancy, mean endometrial thickness was higher compared to cycles with negative outcomes. Higher serum estradiol is associated with higher endometrial thickness and pregnancy rates. Women achieving pregnancy and pregnant women with endometrium thicker than 9 mm were younger. Follicle stimulation was better with higher endometrial thickness. After adjustments for age, no statistical difference was found in endometrial thickness between agonist and antagonist protocols.