Objectives: The aim of the study was to determine the role of maternal and neonatal plasma vitamin D levels in early-onset sepsis (EOS) and in its severity in term infants. Methods: This study was performed on 60 neonates and their mothers who were divided into two groups. Group 1 comprised 30 full-term neonates with EOS. Group 2 comprised 30 healthy neonates of matched age and sex who served as the control group. All participants were subjected to the following investigations: complete blood count, C-reactive protein, blood culture and serum vitamin D levels in both neonates and their mothers. Results: Maternal and neonatal vitamin D levels were lower in the study group compared with the control group and their levels were significantly higher in the summer season. We found a significant negative correlation between infant vitamin D level and C-reactive protein in the patient group. In contrast, there was a significant positive correlation between maternal and neonatal vitamin D levels in both groups. The best cutoff value of neonatal vitamin D for the risk of sepsis was 14.4 ng. Conclusion: Lower maternal and neonatal vitamin D levels are associated with EOS. However, there was no significant relation between vitamin D deficiency and outcome.