An ovarian hyperstimulation syndrome (OHSS) in spontaneous pregnancies is a very rare event. Hence, clinicians might make wrong decisions, such as laparotomy, because of suspicion of an ovarian carcinoma, or severe complications such as renal insufficiency may develop because the diagnosis and treatment of OHSS are delayed. Here we report a case of a woman in the 15th week of gestation, presenting with a partial hydatidiform mole and a triploidy of fetus and placenta, with an ongoing and severe OHSS even after legal induced abortion. A low vascular endothelium growth factor (VEGF) concentration (50 ng/ml) was measured when human chorionic gonadotrophin (HCG) exceeded 1000 IU/l and was followed by VEGF concentrations >900 ng/ml, when the OHSS developed. The literature on spontaneous pregnancies associated with OHSS is reviewed, and possible reasons for the clinical course presented in this study and the reviewed reports are discussed. The findings in this case contribute to our theory that VEGF is a causative factor of OHSS, but has no impact on the course of this disease.