The ovarian hyperstimulation syndrome (OHSS) is the most important complication of the pharmacological ovulation induction. Exclusively postovulatory, it is clinically characterized by a massive ovarian enlargement associated with an acute third-space fluid shift responsible for the development of ascites, and sometimes pleural and/or pericardial effusion. While mild OHSS has no consequence, severe forms can be life-threatening because of associated hemodynamic troubles. The main risk factors are the polycystic ovarian syndrome and an explosive ovarian response to the stimulation characterized by high serum oestradiol levels and an increased number of follicles. Ultrasound and endocrine monitoring make prevention measures possible, mainly by either abandoning the stimulation cycle or, during in vitro fertilization, cryopreserving the embryos for subsequent replacement in another cycle. Treatment consists in correcting circulatory and electrolyte imbalance. Paracentesis is more and more systematically proposed in the severe forms.