The overall incidence of clinically important (moderate to severe) OHSS ranges from 1% to 10% of IVF cycles, but only a small proportion (0.5% to 2%) of the cases are severe. In extreme but rare cases, secondary complications such as deep vein thrombosis, respiratory distress and acute hepato-renal failure may occur. The main risk factors are the presence of polycystic ovaries, high ovarian response to superovulation therapy, the use of hCG to trigger the ovulatory process or for luteal phase support, and the endogenous production of hCG by an early pregnancy. The pathogenesis of OHSS is unknown, although the predominant biochemical mediator is thought to be the renin-angiotensin system. Ovarian stimulation should always be carefully monitored to identify those women at risk. In IVF cycles, the hCG injection should be withheld if the risk is judged to be too great. Some women will benefit from a policy of proceeding to collect oocytes, but electively cryopreserving any resulting embryos, thus allowing the ovarian stimulation cycle not to be wasted. The administration of albumin at the time of oocyte collection will reduce the chance of severe OHSS occurring. If a decision is made to proceed with oocyte recovery and embryo transfer, it may be advisable to give 5000 IU of hCG, rather than 10,000 IU, as the ovulatory trigger. Progesterone, and not hCG, should be given in the luteal phase. Women developing mild or moderate OHSS should be kept under outpatient surveillance to detect the minority that may progress to severe OHSS. Those with severe OHSS should be hospitalised for fluid and electrolyte management. Paracentesis under ultrasound guidance is recommended where there are tense ascites, but further surgical intervention should rarely be undertaken and only when there is good clinical evidence of ovarian torsion or haemorrhage.