Preterm infants with respiratory distress who are <37 weeks of gestational age and will be admitted to Neonatal Intensive Care Unit of Kanuni Sultan Suleyman Training and Research Hospital will be included in this prospective study. Blood for neonatal and maternal vitamin D levels will be obtained from all infants and their mothers at the time of hospital admission. The infants will be evaluated for RDS and surfactant requirement. If they have RDS and need of mechanical ventilation, they will be intubated and given surfactant, if necessary. The maternal and neonatal demographics of infants will be recorded. Infants will also be followed up in terms of other morbidities including patent ductus arteriosus, necrotizing enterocolitis, retinopathy of prematurity, and especially BPD. Then infants will be divided into three groups according to maternal/neonatal vitamin D levels (mild, moderate and severe vitamin D deficiency) and group comparisons will be performed Plasmas after separated and stored at -80 C. Levels of 25-OHD will be determined using Shimadzu LC-20AT model High Performance Liquid Chromatography (HPLC) system. Data will be analyzed using SPSS software and appropriate statistical analyses will be performed.