On the basis of these considerations, we hypothesized that the occurrence of severe complications in preterm infants—RDS requiring mechanical ventilation (MV), BPD, IVH and ROP—might be affected by polymorphisms in genes coding the VEGFA, eNOS enzyme, RAS system (Angiotensinogen gene [AGT], AGTR1,ACE), and HMOX-1 enzyme. This evidence concerns the gene NOS3 and retinopathy of prematurity.