Preterm ovarian hyperstimulation syndrome (POHS) is a rare condition in which immaturity of the gonadal axis is accepted as the cause. Based on our case and 8 cases from the literature, we made an attempt to specify the mechanisms underlying the syndrome and its pathognomonic signs. All POHS newborns were born between 24 and 31 weeks postconception age (WPCA) and developed vulvar, hypogastric and upper leg swelling, and ovarian follicular cyst/cysts (10–40 mm in diameter) with mildly or considerably elevated E2 concentrations (80–5,300 pmol/l) between 35 and 39 WPCA. The GnRH test, performed in 5 cases, confirmed gonadal axis activation. In our case the observed normalization of elevated gonadotropin values by 43 WPCA, accompanied by a simultaneously increasing E2 value (approximately 800 pmol/l), could correspond with the maturation of the gonadal steroid-dependent negative-feedback mechanism. The continuously increasing E2 levels after this period (maximum 1,300 pmol/l) suggest its autonomous secretion. In all the cases, including 3 neonates treated with medroxyprogesterone and surgery, the swelling resolved by 6 months. Conclusions: A pathognomonic sign of POHS is swelling which develops around 37 ± 3 WPCA, and the syndrome is only infrequently diagnosed when the swelling is profound. The direct etiologic factor here is not E2. POHS does not require therapy as long as there is no danger of cyst torsion.