Ovarian hyperstimulation syndrome (OHSS) is the most serious iatrogenic complication of ovarian stimulation. In severe cases, haemoconcentration, hypovolaemia, thromboembolism and death may result. It is reassuring that its incidence is not increased after ovarian stimulation for in-vitro fertilization despite very high serum oestradiol levels and large numbers of follicles and oocytes. This may be related to follicular aspiration, expert monitoring or low implantation rates. However, complete prevention has not been achieved despite the wide availability of ultrasound and oestradiol assays, thus presenting the clinician with a continuous challenge. Our aim is to analyse critically the most recent published series of OHSS in in-vitro fertilization and other assisted reproduction techniques using stimulation with gonadotrophin releasing hormone agonists (GnRHa) and human menopausal gonadotrophin (HMG). The main determining factor in the development of OHSS appears to be ovarian predisposition. Patients with polycystic ovarian disease are at a high risk of OHSS and therefore a small dose and slow start of HMG is recommended, tailoring the dosage according to the ovarian response. Accurate prediction by ultrasound and oestradiol assays and strict prevention by withholding human chorionic gonadotrophin (HCG) or cryopreservation of all the embryos have a major impact on the occurrence of OHSS. It is interesting that fixed-schedule IVF cycles, without detailed monitoring, are not associated with an increased incidence of OHSS. The use of GnRHa, despite expectations, is associated with a higher prevalence of OHSS. Luteal phase supplementation with progesterone rather than HCG should be used in cycles where oestradiol is greater than 2500 ng/l or where the number of oocytes exceeded 10.(ABSTRACT TRUNCATED AT 250 WORDS)