Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family and plays an important role in pathological angiogenic events. PlGF exerts its biological activities through binding to VEGFR1, a receptor tyrosine kinase that consists of seven immunoglobulin-like domains in its extracellular portion. Here we report the crystal structure of PlGF bound to the second immunoglobulin-like domain of VEGFR1 at 2.5 A resolution and compare the complex to the closely related structure of VEGF bound to the same receptor domain. The two growth factors, PlGF and VEGF, share a sequence identity of approximately 50%. Despite this moderate sequence conservation, they bind to the same binding interface of VEGFR1 in a very similar fashion, suggesting that both growth factors could induce very similar if not identical signaling events.