During assisted reproduction technologies, controlled hyperstimulation of the ovaries occurs. Ovarian hyperstimulation syndrome is an excessive overreaction of the ovaries complicating pharmacological ovulation induction. Rarely other causes, such as the mutation of the follicle-stimulating hormone receptor may also be in the background. Ovarian hyperstimulation syndrome 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 or pericardial effusion. Associated arterial or venous thromboembolic symptoms are also common. Ovarian hyperstimulation syndrome is an iatrogenic and potentially life-threatening condition in the form of ischemic stroke or circulatory insufficiency of the limbs. Recently some new methods have been developed for the prevention of the disease. The syndrome affects young, healthy patients. It also has an important economic burden due to the absence from work, bed rest, or hospitalization and intensive medical management of more severe cases. Supportive therapy, anticoagulant prophylaxis and close monitoring are the main approach for the syndrome. However, hospitalization or intervention should not be delayed for patients with severe or critical conditions. Orv Hetil. 2018; 159(34): 1390–1398.