Introduction: This study aims to determine the prevalence of vitamin D deficiency/insufficiency according to age, gender, and seasonal variables in children admitted to a tertiary hospital. Methods: A cross-sectional study was conducted in the pediatric clinic of a tertiary care hospital in Ankara. Healthy children whose 25-hydroxyvitamin D levels were measured for any reason from January 2015 to January 2020 were included in the study. The vitamin D levels of the participants were analyzed according to age, gender, and measuring season. Children were divided into 4 subgroups according to their age: infancy (0-1 years), preschool (2-5 years), school (6-11 years), and adolescence (12-18 years). 25-hydroxyvitamin D less than 12ng/ml was defined as a deficiency, 12-19 ng/ml were insufficiency, 20-50 ng/ml sufficiency and more than 50ng/ml were defined as the risk of potential adverse effects Results: A total of 16321 children were included in the study. The participants detected Vitamin D deficiency in 5243 (32.1%). The deficiency was mostly detected during adolescence (n:3313, 63.2%). This was followed by the school period (23.5%), preschool period (10.5%), and infancy (2.7%), respectively. The deficiency was mostly detected during the winter months. While vitamin D deficiency was more common in females over the age of six, no difference was found between the sexes under the age of six. Conclusions: In this study, a high frequency of vitamin D deficiency/insufficiency was found in children. We believe that screening programs to be determined according to age groups can be beneficial to realize this deficiency without delay.