Spontaneous forms of the ovarian hyperstimulation syndrome (sOHSS) are nearly always reported between 8 and 14 weeks of pregnancy and also with follicle-stimulating hormone (FSH) producing pituitary adenoma. The syndrome has been previously reported in rare instances of increased production of human chorionic gonadotrophin (hCG) such as multiple pregnancies, hydatiforme mole, polycystic ovary disease and elevated concentrations of thyroid-stimulating hormone (TSH) in hypothyreoidism. High levels of these hormones are able to stimulate by natural promiscuous activation the wild-type FSHr, resulting in sporadic presentations of the syndrome. Since 2003, only six different activating FSHr gene mutations have been reported in cases of familial or habitual sOHSS. In addition to five mutations which have been found in the transmembrane helices (Asp567Asn, Asp567Gly, Thr449Ile, Thr449Ala, Ile545Thr), the first germline mutation (c.383C > A, p. Ser 128 Tyr) in the extracelullar domain was identified. All five mutants were abnormally activated by TSH and normal levels of hCG while displaying constitutive activity. In contrast to these mutations, the p.Ser128Tyr mutant displayed an increase in sensitivity only toward hCG. Accordingly, the mutated FSHrs, may be hyperstimulated by the pregnancy-derived hCG or TSH, inducing the occurrence of the syndrome. In the differential diagnosis, malignancy, pregnancy luteoma and hyperreactio luteinalis would have to be excluded. In almost all of the cases the disease regresses spontaneously and could be managed expectantly or conservatively, but with termination of pregnancy or surgery in cases of complications.