Background: Arginine deficiency contributes a significant metabolic problem in preterm babies especially those with necrotizing enterocolitis (NEC). A [C-to A-] nucleotide transverse in the gene encodes Carbamyol-Phosphate Synthetase 1 (CPS1) enzyme has been associated with low arginine level among term neonates but not in adult. Aim of the study: We aimed at investigating the association between CPS1 gene (4217 C>A) polymorphism and plasma L-arginine level among Egyptian preterm infants with NEC. Material & methods: A case-control study was conducted on 30 preterm infant (26-34 weeks of gestation) with NEC. Thirty sex and gestational age harmonized preterm babies without NEC were eligible as controls. Both cases and controls were subjected to plasma L-arginine level measurement using ELISA and genotyped for CPS1 gene (4217 C>A) using PCR-based RFLP-assay. Results: There were significant decrease in plasma L-arginine concentration in preterm with NEC when compared to controls (P=0.002).The frequency distribution of CPS1 gene (4217 C>A) genotypes were in agreement with Hardy Weinberg (HW) equilibrium. The distribution of CPS1 gene (4217 C>A) C/C , C/A, A/A genotypes and A allele were 20(66.6%),8(26.7%), 2(6.7%) and12(20%) among cases and 22(7.3%),6(20%),2(6.7%) and 10(16.7%) among controls with estimated odds ratio of 1.46 ,1.1 and 1.25 respectively. There was no significant relation between low L-arginine level and CPS1 gene (4217 C>A) genotypes. A significantly lower L-arginine levels were observed among NEC non-survivors when compared to survivors (P=0.004), however, there was no significant difference between 2 groups regarding CPS1 gene (4217 C>A) genotype distribution. (P=0.570). Conclusion: A significantly lower L-arginine level was detected among preterm with NEC and significantly related to poor outcome. However, no significant relation was observed between low L-arginine level and CPS1 gene (4107 C>A) polymorphism.