In this prospective study the concentration of circulating vascular endothelial growth factor (VEGF) was followed in 10 patients with severe ovarian hyperstimulation syndrome (OHSS) after ovarian stimulation and in 15 patients without OHSS. VEGF was assayed by means of two different commercially available kits as either free or total VEGF in serum. The concentration of free VEGF was significantly higher on the days of human chorionic gonadotrophin (HCG) administration (309.4 ± 165.0 versus 190.3 ± 127.8 pg/ml, P < 0.05) and embryo transfer(315.0 ± 125.2 versus 209.3 ± 137.2 pg/ml, P < 0.05) in the OHSS compared to the control group. No such difference existed with respect to total circulating VEGF. In addition, there was no significant rise in the free or in the total serum VEGF concentration in the OHSS patients or the controls from the day of HCG administration up to the days of oocyte retrieval or embryo transfer. A cut-off concentration of 200 pg/ml free serum VEGF concentration on the day of HCG treatment resulted in a sensitivity of 90% and a specificity of 80% for the prediction of OHSS development. This is the first report on the parallel measurement of free and total VEGF in serum following ovarian stimulation. The value of the proposed cut-off concentration should be confirmed in a study of a larger group of women.