Aim: Current studies show that vitamin D deficiency is associated with many conditions such as: autoimmune diseases, malignancies, cardiovascular diseases, etc. The aim of this study is to determine the frequency of vitamin D deficiency in children who applied to the pediatric outpatient clinics of our hospital between 2012-2019. Method: In this cross-sectional study, 1909 participants whose vitamin D measurements were made during application for any reason to pediatric outpatient clinics between 2012-2019 were included. All of the data was obtained from hospital infomation systems. 25-(OH)-D3 level was classified as low (below<12 ng/mL), vitamin D deficiency (12-20 ng/mL), normal (20-100 ng/mL) and vitamin D intoxication (>100 ng/ml). SPSS 21 program was used for statistical analysis. P<0.05 was considered significant. Results: Vitamin D deficiency was found in 9,0% of the participants and 22,9% in vitamin D deficiency. Vitamin D deficiency and insufficiency were found to be statistically significantly lower in winter and spring compared to summer and autumn, in girls compared to boys, and in the 2-12 age group compared to the 0-2 age group (p=0,000; p=0,000; p=0,000, respectively). Considering the diagnostic codes, vitamin D deficiency was found most frequently when investigating the diagnosis of infectious diseases. Conclusion: It has been observed that the prevalence of vita-min D deficiency in children increases by years and with increasing age. Adopting a preventive life style from vitamin D deficiency in children and evaluating them in the interim periods may be beneficial in terms of preventing possible complications.