OBJECTIVE To test whether elevated hCG alone triggers spontaneous ovarian hyperstimulation syndrome (sOHSS). METHODS Retrospective analysis. METHODS Departments of obstetrics and gynecology and of medical genetics in an academic medical center. METHODS A patient with sOHSS and 109 patients with elevated hCG. METHODS Collecting blood samples. METHODS Follicle-stimulating hormone receptor gene sequence, levels of TSH and hCG. RESULTS We described a case of sporadic, nonfamilial sOHSS. Sequencing of the entire coding region the FSH gene revealed wild-type alleles for all the known mutations, and the A919G and A2039G polymorphisms, previously associated with good response to FSH stimulation and severe iatrogenic OHSS. We ruled out hypothyroidism. The level of hCG reached a peak of 344,350 IU/L (95th percentile). One hundred nine pregnancies with hCG of >150,000 IU/L were identified from 2001-2006. After patients with gestational trophoblastic diseases, multiple pregnancies, and iatrogenic OHSS were excluded, 27 patients remained. None of those patients experienced OHSS. CONCLUSIONS Elevated hCG cannot be responsible for sOHSS as a single factor. Factors other than the hCG-FSH-receptor interaction additionally are involved in the pathogenesis of this syndrome. A combination of mechanisms may allow understanding of this enigmatic disorder. The pathophysiology of sOHSS, a rare phenomenon, has not yet been elucidated.