Recent studies suggest that vitamin D insufficiency or deficiency is not only a problem of older generations anymore but also an important health concern among younger generations. However, comprehensive data are lacking in Korean adolescents. We investigated the vitamin D (25-hydroxy vitamin D [25(OH)D]) status, the prevalence of vitamin D insufficiency or deficiency, and the association between vitamin D levels and insulin resistance and lipid profiles in a sample of 188 Korean adolescents aged 12-13 years who participated in a general health check-up at a tertiary hospital. Vitamin D deficiency was considered as serum concentrations <20 ng/mL (50 nmol/L); a level of 21-29 ng/mL (52-72 nmol/L) was considered to indicate vitamin D insufficiency, whereas a level of 30 ng/mL or greater (>75 nmol/L) was considered sufficient or optimum. In this cross-sectional study, vitamin D insufficiency or deficiency was found in 98.9 % of boys and 100 % of girls, whereas only 1.1 % of boys and 0 % of girls had a serum 25(OH)D level of greater than 30 ng/mL. In multivariate linear regression analysis, HOMA-IR, triglyceride, and LDL cholesterol were inversely associated with 25(OH)D concentrations. We found that vitamin D insufficiency or deficiency is a very common health problem in Korean adolescents, particularly in girls, and that serum 25(OH)D levels are inversely associated with insulin resistance and lipid profiles. These results suggest that more time spent in outdoor activity for sunlight exposure and higher vitamin D intake may be needed in younger adolescents in South Korea.