['To explore the value of serum vascular endothelial growth factor (VEGF) concentrations during IVF cycles in predicting the risk of ovarian hyperstimulation syndrome (OHSS).', 'Prospective study.', "London Women's Clinic.", 'One hundred seven women undergoing IVF. Mild OHSS developed in 10 women, moderate OHSS in 7, and severe OHSS in 3.', 'Serum VEGF concentrations were measured before treatment, after pituitary desensitization, and on the days of hCG administration, oocyte collection, and ET.', 'Serum VEGF concentrations.', 'Serum VEGF concentrations were higher in women in whom OHSS developed. The increase in the VEGF concentration that occurred between the day of hCG administration and the day of oocyte collection (the "VEGF rise") was an important marker of OHSS. The VEGF rise was higher in women in whom OHSS developed. A higher VEGF rise predicted all cases of OHSS and moderate/severe cases of OHSS with a sensitivity of 100% and a specificity of 60%. A likelihood ratio test showed that adding the VEGF rise or the VEGF concentration on the day of oocyte collection to a regression model as a continuous variable to the number of follicles, the E2 concentration, and the presence of polycystic ovaries significantly contributed to predicting the risk of OHSS.'] Do serum vascular endothelial growth factor concentrations in in vitro fertilization cycles predict the risk of ovarian hyperstimulation syndrome?