Background: Previous studies have shown the relationship between in utero lung development and vitamin D [25(OH)D], but there have been no studies to investigate whether vitamin D deficiency is a risk factor for respiratory distress syndrome (RDS) in preterm babies. Objectives: In this study, we investigated if 25(OH)D deficiency is a risk factor for RDS. Methods: One hundred fifty-two preterm newborns, born at 29-35 weeks gestational age, were included in the study following informed consent from the parents. Peripheral blood samples were collected within the first 24 hours of life and 25(OH)D levels were measured by liquid chromatography-tandem mass spectrometry. Demographic characteristics of the babies and the diagnosis of RDS were recorded. Results: In 64% of preterm infants, 25(OH)D levels were compatible with severe deficiency (<= 10 ng/mL), 33 % with moderate deficiency (10-20 ng/mL), and 3 % with mild deficiency (20-30 ng/mL). In none of the babies was a normal 25(OH)D level observed. Serum 25(OH)D levels were not correlated with gestational age. Respiratory distress syndrome was more common in preterm babies with severe (28 %) compared to mild-moderate 25(OH)D deficiency (14 %) (p<0.05). Conclusions: None of the preterm infants in this study had normal vitamin D level, which underlined the burden of vitamin D deficiency in pregnant women and their offspring. RDS was more common in severely vitamin D-deficient preterms. Determination of vitamin D status of the mothers and appropriate supplementation might be a valuable strategy to reduce RIDS, in addition to antenatal steroids. Besides, since vitamin D is a regulatory factor in many organs during fetal development, long-term effects of in utero vitamin D deficiency warrant further studies.