Objective : to compare the level of peripheral blood natural killer (NK) cells (CD3, CD56, CD16) in cases of RIF and women with history of at least one successful ICSI trial.   
Methods : A prospective cohort study conducted on 50 women underwent ICSI trial classified into 2 groups. Group I included 25 women repeated (2 or more) ICSI failures and group II included 25 women who previously achieved clinical pregnancy at least once in previous trial. Peripheral blood NKCs was assessed in all women. The primary outcome parameter was the number of NKCs in women of both groups. Other outcomes included the number and quality of retrieved oocytes, the number and quality of obtained embryos, fertilization rate, implantation rate, chemical pregnancy rate, clinical pregnancy rate, ongoing pregnancy rate and live birth rate.  
Results: No significant difference between women with previous IVF failure and those with previous IVF success regarding number and quality of retrieved oocytes, number and quality of embryos, fertilization, implantation, clinical pregnancy, ongoing pregnancy and live birth rates. However, the chemical pregnancy rate was significantly higher in women with previous IVF success.  
Conclusion: NKCs (CD56. CD16.CD3.) levels did not differ significantly between recurrent implantation failure cases and recurrent successful implantation controls.