Currently, there is a tendency toward increasing usage of criocycles in programs of in vitro fertilization. This approach allows to achieve pregnancy as well as prevent the development of ovarian hyperstimulation syndrome and utilize the pre-implantation genetic diagnostics. The objective: to improve the efficiency of in vitro fertilization treatment cycles with transferring of cryopreserved/warmed embryos into the uterus. Patients and methods. The study of 824 treatment cycles (in natural menstrual cycle, in cycle with hormone replacement therapy, after egg donation, in natural cycle without embryo transfer in «fresh» cycle, in cycle with hormone replacement therapy without embryo transfer in «fresh» cycle). Vitrification and cryopreservation method. Results. The analysis of the study results has shown that the implantation rate of frozen embryos in the natural cycle is higher than in the cycle with hormone replacement therapy. The implantation rate of frozen embryos after the use of agonist gonadotropin-releasing hormone as a trigger of final maturation of the agonist is higher than that of human chorionic gonadotropin, which allows us to consider vitrification as an effective tool that provides great flexibility of conducting controlled ovarian stimulation cycles and prevention of ovarian hyperstimulation syndrome. Conclusion. The effectiveness of frozen embryo transfer protocols gives us grounds to assert that the optimum cycle for the transfer is a cycle without stimulation. Key words: in vitro fertilization, the endometrium, frozen embryo implantation.
