Ovarian hyperstimulation syndrome (OHSS) is a rare and potentially life-threatening complication during controlled ovarian stimulation. It can be associated with severe morbidity and may even be fatal. The etiology of the condition and predisposing factors are still not fully understood. Data concerning pathophysiology in patients with OHSS were searched using PubMed and other medical data bases. The incidence of severe OHSS, as calculated by World Health Organization (WHO), is 0.2-1% of all stimulation cycles in assisted reproduction. Considerations on OHSS classifications and forms of manifestations are discussed in detail. New insights concerning genetics and altered FSH receptor are given. OHSS may involve, according to its grade of severity, elevated or decreased levels of growth factors, cytokines, mediators, changes in hormones, renin-angiotensin and kinin-kallikrein system. There are massive electrolytic imbalances and changes in hemodynamic and fluid metabolism. Furthermore, liver and pulmonary dysfunction is observed as well as increased coagulation with subsequent thromboembolism. The influence of OHSS on the pregnancy rate and outcome of pregnancy is a matter of controversy. Patients with OHSS have high pregnancy rates with a tendency to an increased incidence of abortion.