PURPOSE: An adequate vitamin D level is important for normal growth, cancer prevention, controlling hormones, and immune regulation. However, no study has investigated vitamin D status in patients with serious illnesses, such as malignancies, malabsorption diseases, or prematurity in Korean children. Thus, we analyzed the results of 25-hydroxyvitamin D3 (25-D) levels in children in a single tertiary medical center. METHODS: Children who had their vitamin D level evaluated at Keimyung University Dongsan Medical Center from January 2004 to December 2014 were included. We reviewed the medical records and laboratory test results. RESULTS: Ninety-three children (male:female = 48:45) who had their vitamin D levels measured were included. The most common reason for measuring vitamin D level was hypocalcemia with or without neurological signs. Among the subjects, 61 children had hypovitaminosis D (25-D < 30 ng/mL). A positive correlation was detected between 25-D and total calcium levels (p = 0.001). Negative correlations were found between 25-D and immunoreactive parathyroid hormone (iPTH) levels (p = 0.002) and between 25-D and alkaline phosphatase (ALP) levels (p = 0.021). Twenty-three subjects (37.7%) had a critical underlying condition, including a hemato-oncologic issue (18.0%), gastrointestinal or nutritional problem (11.5%), and prematurity or low birth weight (8.2%). CONCLUSION: The levels of total calcium, ALP, and iPTH were correlated with 25-D level. An underlying malignancy, malabsorption issues, or prematurity may be risk factors for hypovitaminosis D in children.