Ovarian hyperstimulation syndrome (OHSS) mostly occurs due to excessive FSH stimulation, with final follicle maturation by hCG for IVF. Symptoms usually start 2–4 days after oocyte retrieval; alternatively, endogenous hCG from an implanting embryo can initiate OHSS some days later. Luteinized follicles synthesize vascular endothelial growth factor (VEGF) which induces angiogenesis and increased capillary permeability by inhibition of the cell adhesion molecule vascular endothelial-cadherin (VE-cadherin). This causes fluid leakage from the vascular bed to the third space, resulting in enlarged lutein cysts and ascites. Initial symptoms are bloated abdomen with pain, and in more severe cases dyspnoea, decreased renal and hepatic function and thromboembolism. There is no causal treatment of established OHSS, but there are several prophylactic strategies based on risk evaluation of each patient. In established OHSS, treatment is mainly supportive, by correcting fluid, salt and protein deficits, and low molecular weight heparin for thrombosis prophylaxis.