Objective The aim of the study is to investigate the relation of neonatal and maternal vitamin D and late-onset sepsis (LOS) Study Design One-hundred twenty term neonates along with their mothers were enrolled in this case–control study. Sixty neonates who were admitted in the neonatal intensive care unit by LOS and had not been previously admitted for last 48 hours and did not receive antibiotics or vitamin D were enrolled as cases (sepsis) group. On the other hand, 60 healthy term neonates were referred as control group. Maternal and neonatal serum 25-OH vitamin D levels were assessed in both the cohorts. Results Maternal and neonatal 25-OH vitamin D levels in cases (17.2 and 16.1 ng/mL, respectively) were significantly lower than in controls (22.7 and 21 ng/mL, respectively) p = 0.001. In the study group, the neonatal 25-OH vitamin D was negatively correlated with C-reactive protein and length of hospital stay (r = −0.616 and −0.596, respectively) p