This study was conducted to investigate the possible role of angiogenin in the pathogenesis of ovarian hyperstimulation syndrome (OHSS). The study group consisted of 10 healthy women who developed severe OHSS (group A) following ovarian stimulation by a long protocol of gonadotrophin-releasing hormone analogues/human menopausal gonadotrophin for in-vitro fertilization. A control group B (n = 10) underwent stimulation by the same protocol and did not develop OHSS. Blood samples were taken from group A on day of admission to hospital for treatment of OHSS and, in group B, 1 week after oocyte retrieval. In group A, ascitic fluid was routinely aspirated as a treatment for severe OHSS, and a peritoneal fluid sample was aspirated transvaginally before oocyte retrieval in group B. In group A, the mean serum angiogenin, the mean ascitic fluid angiogenin, the mean serum oestradiol concentration on day of human chorionic gonadotrophin and the mean haematocrit were 8390 +/- 6836 ng/ml, 2794 +/- 1024 ng/ml, 6300 +/- 2450 pg/ml and 46.6 +/- 4.4 respectively, as compared with 234 +/- 91 ng/ml, 254 +/- 105 ng/ml, 1850 +/- 1100 pg/ml and 36.8 +/- 4.6 in group B respectively. The differences between groups were highly significant for all parameters. Angiogenin seems to be strongly associated with the formation of neovascularization responsible for the development of OHSS.