This is a prospective self controlled clinical trial. Women with clomiphene Citrate failure, and thin endometrium were recruited (N = 42). In their 6th (Clomiphene citrate only) cycle, women continued on Clomiphene citrate 100 mg/ day for 5 days, and had sonographic measurement of their endometrial thickness , and Doppler evaluation of their uterine arteries on the day of HCG administration. In 7th cycle, women (N = 36) were given usual dose of Clomiphene citrate supplemented with sildenafil vagina gel (5 gm, containing 50 mg sildenafil) twice daily from cycle day 8 till the day of HCG injection. Endometrial thickness and uterine artery Doppler were measured on the day of HCG administration.