<p>Background: Endometrium undergoes several changes in structure and cellular composition during pregnancy. Granulocyte Colony-stimulating Factor (GCS-F) is an important cytokine with critical role in embryo implantation and pregnancy. The aim of the present study was to evaluate the impact of intrauterine injection of G-CSF in patients who suffer from unexplained recurrent miscarriage (RM).<br /> Methods: In the present randomized clinical trial, a total of 68 patients were randomly allocated into two study groups including intrauterine G-CSF (n=23, 300 <em>&mu;g</em>) injection and control group (n=27, no G-CSF injection). Eighteen out of 68 patients were excluded from the final analysis due to different reasons. All patients were in Ovulation Induction (I/O) cycle. In G-CSF group, intrauterine injection of G-CSF was done twice in the cycle. All enrolled patients were under 40 years old and had at least two unexplained pregnancy losses. Pregnancy was evaluated by titer of &beta;hCG, presence of gestational sac (implantation) and fetal heart rate (clinical pregnancy) was assessed by vaginal ultrasonography. Student&rsquo;s T test and Mann-Whitney U were used for analysis. The p&le;0.05 was determined as statistically significant.<br /> Results: No significant differences were observed between the two study groups when the rates of chemical pregnancy (26.1%<em> vs.</em> 29.6%, p=0.781), implantation (26.1% <em>vs.</em> 22.2%, p=0.750), clinical pregnancy (17.4% <em>vs.</em> 11.1%, p=0.689) and abortion (33% <em>vs.</em> 37.5%, p=0.296) were compared.<br /> Conclusion: In our study, no significant difference was observed between the two study groups when the rates of chemical pregnancy, implantation, clinical pregnancy and abortion were compared.</p>