Abstract. OBJECTIVE: To evaluate the diagnostic value of serum apelin in early-onset neonatal sepsis in full term neonates. Apelin is a proinflammatory adipocyte-derived factor that participates in vascular wall inflammation. STUDY DESIGN: Case-control study was conducted on 60 full term neonates, 30 cases with early-onset neonatal sepsis and 30 healthy matched controls. Complete blood counts, C-reactive protein, blood cultures, plasma lactate, and serum apelin concentrations (measured by enzyme-linked immunosorbent assay) were determined initially at the time of sepsis diagnosis and 4 days after starting treatment for cases. Only basal serum apelin concentrations were measured for control group. RESULTS: Apelin was detected in all neonates and concentrations were positively correlated to sepsis scores, plasma lactate and CRP. Neonates with sepsis had significantly elevated concentrations (8 folds increase) of serum apelin concnetration as compared to controls [median (IQR): 65.16(46.90) and 7.969(11.36) pg/ml, respectively]. Moreover initial serum apelin concentration measured in cases with culture proven neonatal sepsis was significantly higher than those with negative-culture clinical sepsis (mean ± SD: 73.53 ± 31.77 and 45.22 ± 5.9 respectively, p = 0.0001). The best cutoff value of serum apelin to diagnose early-onset neonatal sepsis was 30.225 pg/ml with a sensitivity of 100% and a specificity of 97%. CONCLUSION: Serum apelin may have a diagnostic value in early-onset neonatal sepsis.