This study was designed to detect vascular endothelial growth factor (VEGF) and its soluble receptor (sVEGFR-1) in follicular fluid specimens and to evaluate the importance of sVEGFR-1 with respect to ovarian response to gonadotrophin stimulation. A total of 69 patients was treated for IVF with recombinant human follicle stimulating hormone (FSH). Concentrations of VEGF and sVEGFR-1 were quantified in follicular fluids from oocyte retrievals. Patients were designated to three groups with respect to the number of harvested oocytes: group A, 1-5 oocytes; group B, 6-10 oocytes; group C, >10 oocytes. In group A, 1133 +/- 870 pg VEGF/ml follicular fluid per oocyte were quantified, in group B 426 +/- 262 pg VEGF/ml per oocyte, and in group C 274 +/- 179 pg VEGF/ml per oocyte. Soluble VEGFR-1 concentrations resulted in 1200 +/- 523 pg/ml follicular fluid per oocyte in group A, 255 +/- 193 pg/ml per oocyte in group B, and 79 +/- 69 pg/ml per oocyte in group C. No free sVEGFR-1 could be detected in any follicular fluid. An index to estimate the biological activity of VEGF by dividing VEGF/sVEGFR-1 revealed an increasing availability of VEGF with higher ovarian response to gonadotrophin therapy. In group A this index was 1.03, in group B 1.71, and in group C 3.21. A delicate balance between VEGF and sVEGFR-1 is necessary to allow an adequate ovarian reaction to gonadotrophin therapy. Excess of bio-active VEGF increases the risk for ovarian hyperstimulation syndrome. Excess of sVEGFR-1 results in poor response and goes in parallel with reduced chances for conception.