Background: Unexplained recurrent miscarriage can be a challenging and frustrating condition for both patients and clinicians. Sildenafil citrate is one of the currently available lines of treatment according to simplicity of use, reliability and degree of invasiveness but up to now there are no prospective randomized studies, powerful enough. Aim of this Work: To evaluate the efficacy of oral sildenafil citrate 20 mg on uterine arteries Doppler flow studies in patients with unexplained recurrent miscarriages. Patients and Methods: This prospective case control study was conducted at Sayed Galal Maternity Hospital outpatient clinics on 180 women who' were divided into 2 groups: (group 1) which included 91 patients who took sildenafil citrate 20 mg orally, 4 times per day for 21 days and (group 2) which included 89 patients who took placebo orally, 4 times per day for 21 days in the period starting from (August 2017 to June 2018). Results: The statistical comparison between group I (sildenafil citrate) and group II (placebo) before and after administration of drug regarding endometrial thickness showed non-significant difference before and significant difference after 3 weeks of administration of sildenafil citrate in group I and placebo in group II. The statistical comparison in patients in group I (sildenafil citrate) before and after 1 week, 2 weeks and 3 weeks administration of drug regarding pulsatility index, resistance index and systolic/diastolic ratio showed significant difference. Conclusion: Sildenafil citrate has an effective role in increasing the uterine arteries blood flow in patients with recurrent miscarriage.