Introduction. Granulocyte colony stimulating factor (G-CSF) activates the T helper type 2 immune response, provides tolerance of the mucosal immune system and promotes implantation of the embryo into the endometrium. Purpose. To investigate whether G-CSG affects implantation rates and clinical pregnancy rates in in the modified natural cycle frozen–thawed embryo transfer (NC-FET). Methods. A single-center, open, prospective, randomized, comparative (study group: n=33, comparison group: n=33) cohort study of the clinical efficacy of the combined administration of G-CSF in a modified NC-FET in patients with repeated implantation failure (RIF) was performed. Every embryo underwent preimplantation genetic testing aneuploidy by a-CGH method. G-CSF was given subcutaneously daily at a dose of 6 million units, starting from the first day of the protocol. Single transcervical intrauterine infusion of G-CSF (24 million units) was done on the day of hCG administration. In the event of a biochemical pregnancy, subcutaneous administration of G-CSF continued until 63 days after the day of frozen-thawed embryo transfer. Main results. The clinical pregnancy rate and the live birth rate were higher in the G-CSF group: 69.7 vs 45.5% (χ2=3.93, p=0.047) and 63.64 vs 39.39%, (χ2=3.88, p=0.04), respectively. Authors’ conclusions. The combined use of G-CSF in NC-FET may lead to increase in positive IVF outcomes, especially in patients with RIF.