Variations in Vascular Endothelial Growth Factor (VEGF) levels were prospectively evaluated in 18 young women undergoing in vitro fertilization treatments according to the "Long Protocol" and a typical pattern of VEGF levels was recorded. A significant increase in VEGF concentrations was observed only when the follicles reached a mean diameter of 15.3 mm in concurrence with mature oocyte retrieval. Since an increase in VEGF levels is related to follicular vascularity and oocyte developpment, our study supports the approach that oocyte retrieval may be performed when follicles > 15 mm in diameter appear. Anticipating egg retrieval in young patients with an optimal ovarian reserve may decrease the incidence of severe ovarian hyperstimulation, without compromising the treatment results.