Background: Neonatal respiratory distress syndrome (RDS) is a multifactorial disease of preterm influenced by many factors including gene polymorphism. The aim of the present study was to determine the effect of endothelial nitric oxide synthase gene glu298asp polymorphism (rs1799983) in developing and grading of RDS in preterm neonates. Methods: This study was performed on 65 preterm neonates; 40 with RDS and 25 healthy controls. Genotyping of endothelial nitric oxide synthase gene glu298asp polymorphism was performed by restriction fragment length polymorphism. Results: There were statistical significant increases in TT, (GT+TT) genotypes and T allele frequencies of rs1799983 among RDS neonates compared to controls. As groups were categorized by gestational age; TT genotype and T allele frequencies were statistically significantly increased in 33-35weeks RDS neonates compared to controls. TT genotype in RDS was associated with RDS grade III, mechanical ventilation need, and increased mortality. TT genotype, T allele, gestational age (