Purpose: Recently, vitamin D insufficiency has increased and has been correlated to growth and puberty in children. This study was conducted to find the prevalence of subclinical vitamin D insufficiency and its influence on school-aged children in Korea. Methods: The subjects of this study were 397 children aged 7 to 15 years who had been tested for 25-OH vitamin D3 among the outpatients of the Department of Pediatrics in Eulji General Hospital from March 2007 to February 2011. Data for age, sex, comorbidities, serum 25-OH vitamin D3, height, weight, body mass index (BMI), and sunlight exposure time were collected before and after 3 months of vitamin D administration, retrospectively. Results: Vitamin D insufficiency was present in 343 (86%) of the subjects. In the vitamin D insufficient group, chronological age was $8.96{pm}1.72$ years, mean height (z-score [z]) was $0.51{pm}1.26$, mean BMI (z) was $0.81{pm}2.20$, and bone age was $10.26{pm}1.75$ years. In the vitamin D sufficient group, chronological age was $9.61{pm}1.77$ years, mean height (z) was -$0.66{pm}0.98$, mean BMI (z) was -$0.01{pm}1.16$, and bone age was $9.44{pm}2.12$ years. A paired t-test showed that three months after vitamin D administration, the mean 25-OH vitamin D3 level in the insufficient group increased to $24.38{pm}10.03$ ng/mL and mean BMI (z) decreased to $0.67{pm}1.06$. Conclusion: In Korean school-aged children, vitamin D insufficiency were relatively higher and may be closely related with higher BMI. Insufficient rise of the level of vitamin D after supplementation suggest the new supplementation guidelines, especially for Korean children.