In a retrospective analysis of 637 cycles of ovarian stimulation and transvaginal follicular aspiration for various assisted reproductive technologies, severe ovarian hyperstimulation syndrome (SOH) occurred in six (0.94%) cycles. The patients at a high risk of developing SOH in cycles of assisted reproduction were those who had excessive serum oestradiol levels on the day of human chorionic gonadotrophin (HCG) administration (oestradiol greater than 6000 pg/ml; 38% SOH) and a high number of oocytes obtained (greater than 30 oocytes; 23% SOH). In those patients with both oestradiol greater than 6000 pg/ml on the day of HCG administration and greater than 30 eggs retrieved, the chance of developing SOH was 80%. The higher the serum oestradiol levels and the more eggs retrieved, the higher the pregnancy rates observed. High oestradiol level did not appear to have a detrimental effect on pregnancy rates and outcome. Furthermore, our results are not consistent with suggestions that the addition of gonadotrophin-releasing hormone agonist to ovarian stimulation protocols, follicular aspiration and/or luteal support with progesterone may reduce the incidence of ovarian hyperstimulation syndrome.