We investigated the role of vascular endothelial growth factor (VEGF) in ovarian hyperstimulation syndrome (OHSS). Two similar peaks of permeability activity were seen in OHSS ascites and liver ascites spiked with recombinant human VEGF (rhVEGF); no activity was seen in control liver ascites. Incubation with rhVEGF antiserum decreased activity in the two OHSS peaks by 79% and 65% and the two spiked liver peaks by 49% and 50%. Control serum produced 24% and 27%, and 17% and 0% reductions, respectively. This is evidence that the major capillary permeability agent in OHSS ascites fluid is VEGF.