Cyclic ovarian angiogenesis is critical for normal ovarian folliculogenesis, ovulation, and corpus luteum formation and maintenance. This process is regulated by the VEGF family. Several in vitro studies have suggested that LH/hCG (human chorionic gonadotropin) modulates the expression of ovarian follicular VEGF expression. The aim of this study is to test the hypothesis that rLH supplementation during the late follicular phase of down-regulation protocol for controlled ovarian hyperstimulation improves follicular angiogenesis. A prospective, randomized, double-blind, placebo-controlled study. Pre-menopausal women aged 27-39 years, with body mass index 17-27, undergoing in-vitro fertilization (IVF) treatment (first to third cycle). After down-regulation, patients were treated with recombinant follicle stimulating hormone (rFSH) (225 IU/day). When at least two follicles reached a mean diameter of 14 mm, patients were randomized using a computer-generated randomization list (stratified by Serono Inc.) to rLH (Luveris) (75 IU/day) (n = 10), or placebo (sucrose) (n = 10). Sera and follicular fluid (FF) VEGF-A165, sFlt-1/VEGFR-1 and PIGF protein levels were measured by ELISA. Recombinant LH significantly increased FF VEGF-A165/sFlt-1 ratio (P=0.05), but decreased FF VEGF-A165/PlGF ratio, without statistical significance. Compared to placebo, rLH decreased serum levels of VEGF-A165 and sFlt-1/VEGFR-1, although values were not statistically significant. The PlGF plasma levels were undetectable. This is the first in vivo study to demonstrate that rLH induces ovarian follicular angiogenesis via modulation of VEGF-A165 and its soluble receptor sFlt-1/VEGFR-1 expression. Therefore, rLH supplementation during the late follicular phase of down-regulation protocol for controlled ovarian hyperstimulation in patients undergoing IVF may potentially be biologically beneficial in terms of IVF treatment outcomes.