A cohort study was undertaken to compare the effect at the time of oocyte retrieval of the i.v. administration of either 1000 ml of lactated Ringer's solution or 1000 ml of a 5% solution of human albumin on in-vitro fertilization patients at risk for severe ovarian hyperstimulation syndrome (OHSS). A total of 207 patients with an oestradiol concentration > 10,000 pmol/l and/or > 15 follicles (> 10 mm diameter) on the day of human chorionic gonadotrophin (HCG) injection were reviewed. Of these, 158 women received 500 ml of lactated Ringer's solution both before and after egg retrieval, and 49 women received two infusions of 500 ml of 5% human albumin in normal saline at the time of egg retrieval. Severe OHSS developed in two patients who received human albumin and in 10 women who did not receive the albumin. This difference was not statistically significant. There were no differences between the two groups in terms of age, number of follicles punctured at transvaginal oocyte retrieval or oestradiol concentration at the time of HCG injection. The administration of a 5% human albumin solution does not prevent the development of severe OHSS in at risk patients. It does appear to blunt the severity of the condition.