Ovarian hyperstimulation syndrome (OHS) is the most serious complication of ovulation induction, particularly in in vitro fertilization. It is a potentially life-threatening situation. Its pathophysiology is poorly understood. This syndrome is explained by a sudden increase in capillary permeability which results in a rapid fluid shift from the intravascular space into a third space leading to haemodynamic changes. In its most severe forms. OHS is characterized by multicystic ovarian enlargement, hemoconcentration, hypovolemia, oliguria, third space accumulation of fluid in the form of ascites and pleural effusion, renal failure, thrombotic disorders. Mild and the most of moderate forms of OHS usually do not require any active form of therapy. Severe OHS requires hospitalization, correction of fluid and electrolyte imbalance, prevention of thromboembolism, aspiration of the ascites and pleural effusion causing respiratory discomfort and dyspnea. Surgical interventions are exceptionally indicated and reserved for ovarian or rupture of ovarian cyst. Although severe OHS may not be completely avoided, early recognition of high-risk factors, judicious monitoring of ovulation induction (plasma estradiol levels and ultrasonography), and, perhaps in future, substitution of hCG for triggering ovulation should reduce the incidence of this iatrogenic syndrome.