BACKGROUND Progesterone is an endogenous hormone secreted by ovaries that prepares uterine endometrium for implantation. The aim of this study was to evaluate the relation of serum progesterone level on the day of transferring frozen embryo with pregnancy occurrence. MATERIALS AND METHODS 161 women were chosen for frozen embryo transfer. From the second day of their cycle, they were treated with 2 mg per 8 hours estradiol. From the 13th day, 5 mg intramuscular (IM) progesterone was injected every 12 hours to increase endometrium thickness above 8 mm. 4 days after progesterone administration, embryo transfer was performed, and progesterone level was measured. Pregnancy occurrence was also evaluated.