Neonatal sepsis is a clinical syndrome characterized by signs and symptoms of infection with or without accompanying bacteremia in the first month of life which is still an important cause of morbidity and mortality. Vitamin D is a fat-soluble steroid hormone that primarily contributes to the maintenance of normal calcium homeostasis and skeletal mineralization. In addition to its classical role in bone metabolism, vitamin D also has immunomodulatory effects on immune function. Although some studies reported a link between vitamin D deficiency and critical illness in adults, a direct relationship has not been directly shown yet. However, to the best of our knowledge, no study evaluated the association between EOS and maternal/neonatal vitamin D levels. The objective of this prospective study is to determine the possible role of maternal and neonatal plasma vitamin D levels on EOS development in term infants. We also aim to evaluate possible effect of the severity of vitamin D deficiency on EOS development in the study population.