Ovarian hyperstimulation syndrome (OHSS) is brought about by overstimulating the ovaries with gonadotrophins, whether during ovulation induction or so-called controlled ovarian hyperstimulation before intra-uterine insemination (IUI) or IVF. It is a purely iatrogenic (physician induced) condition which is largely preventable and often foreseeable. Those at risk to develop OHSS are young, lean and/or have polycystic ovaries, high serum AMH concentrations and a high antral follicle count and patients who have had OHSS in a previous cycle. For ovulation induction only a low-dose gonadotrophin protocol should be used as this will eliminate the incidence of OHSS. For IVF in patients predicted to be at high risk for OHSS, a GnRH antagonist protocol is recommended.with the use of a GnRH agonist trigger. OHSS does not occur if hCG is withheld and it may be classified as early, due solely to the hCG injection or late, due to the added effect of hCG secreted by the developing pregnancy. Monitoring, according to the severity of the symptoms, should include fluid and electrolyte balance with detailed recording of fluid input and output, measurement of the extent of intravascular volume decrease including frequent measurements of haematocrit and arterial pressure. Central venous pressure measurement is essential for the more severe cases. Baseline renal and liver function tests should be monitored. Treatment is supportive, according to severity, mainly re-hydration and the maintenance of intravascular fluid volume and blood volume expanders when indicated. Relief of ascites and pleural effusions may be necessary.