The ovarian hyperstimulation syndrome (OHSS) is a complication of controlled ovarian hyperstimulation (COH) protocols performed in women undergoing assisted reproductive technologies. This syndrome is characterized by multiple intra-ovarian corpus luteum and constitution of a third space that can lead to a life-threatening situation. Although the pathophysiology remains unclear, vascular endothelial growth factor (VEGF) and other cytokines, secreted under the influence of exogenous gonadotrophins administered for COH, are involved in increasing capillary permeability. The clinical course varies from increased size of the ovaries to anasarca with potentially fatal circulatory dysfunction. Mortality rate, though not accurately quantified, is significant (1/45 000 to 1/500 000) and mostly due to thromboembolic complications. The only effective treatment is prevention, by adapting ovarian stimulation protocols to OHSS risk factors. There are no specific treatments and therapy is mainly symptomatic until the condition resolves spontaneously.