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Background Ovarian hyperstimulation syndrome (OHSS) is a complication of controlled ovarian hyperstimulation (COH). It is a potentially life-threatening condition that usually occurs either after human chorionic gonadotropins (hCG) administration in susceptible patients or as a result of an implanting pregnancy, regardless of whether it was achieved by natural conception or infertility treatments. Despite many years of clinical experience regarding the adoption of preventive measures and the identification of patients at high risk, the pathophysiology of OHSS is poorly understood and no reliable predictive risk factors have been identified. Cases presentation We report about two unexpected cases of OHSS following infertility treatments, occurring after freeze-all strategy with embryo cryopreservation approaches. The first case developed spontaneous OHSS (sOHSS), despite efforts to prevent its manifestation by a segmentation approach, including frozen embryo replacement cycle. The second case developed a late form of iatrogenic OHSS (iOHSS), even though the absence of any risk factors. No mutations in the follicle-stimulating hormone (FSH) receptor (FSHR)-encoding gene were detected, suggesting that the high levels of hCG due to the twin implanting pregnancies could be the only triggering factor of OHSS outbreak. Conclusion Freeze-all strategy with embryo cryopreservation cannot entirely prevent the development of OHSS, which may occur in its spontaneous form independently from the FSHR genotype. Although OHSS remains a rare event, all infertile patients requiring ovulation induction or controlled ovarian stimulation (COS) may be at potential risk of OHSS, either in the presence or in the absence of risk factors. We suggest closely monitoring cases of pregnancy following infertility treatments in order to provide early diagnosis and adopt the conservative management.