Introduction: Embryo cryopreservation is a method of medically assisted fertilization that complements the fresh process of in vitro fertilization and embryo transfer. Excess embryos, formed by in vitro fertilization, are frozen at -196 C and stored in liquid nitrogen. At the appropriate time, treatment is continued by thawing the embryo and transferring it to the uterus at the appropriate stage of the menstrual cycle. Cryopreservation freezes biological material and at the same time prevents its biological activity, which includes cell death. Aim: To determine the success of treatment with cryopreserved embryos, in the first years after the introduction of the method at the Department of Human Reproduction of the Clinical Hospital Center Rijeka. Patients and methods: The research is a retrospective analysis of a medical database over two calendar years (2013. - 2014.). The results of the effectiveness of treatment, that is clinical pregnancies with regard to the age of the patients, the day of ovulation, the day of embryo transfer, the stage of embryo freezing and the medication that the patients received were analyzed and processed. Results: After the transfer of frozen embryos, the effectiveness of treatment is higher in natural cycles, with a shorter follicular phase and if the transfer of the heated embryo is done before the 19th day of the cycle. The efficacy of FET treatment per cycle is 22.4 % corresponding to natural fecundability. Conclusion: The probability of conception after FET corresponds to natural monthly fecundability. During the first two calendar years since the introduction of the method, the success of treatment increases, which is associated with gaining practical experience, studying and better knowledge of embryo cryopreservation methods.