p＞ b＞Objectives of the study/b＞: There was a recent report on the successful treatment of diarrhea-predominant irritable bowel syndrome (IBS) with high dose of oral vitamin D supplement. Vitamin D could improve various symptoms of IBS through its beneficial effect on inflammation and psychological factors. So, the object of this study is to determine the relationship between vitamin D level and the severity of IBS symptoms that may affect the patients’ quality of life. br＞ b＞Methods/b＞: 129 adolescents aged 10-17 years old (70 boys and 59 girls, mean age 16.3±6.6 years old) were included in this study from 2014 to 2016. IBS patients were diagnosed by ROME III criteria and classified to three types by clinical manifestation: IBS with constipation (IBS-C, n=30), IBS with diarrhea (IBS-D, n=66), and IBS with mixing constipation and diarrhea (IBS-M, n=33). Severity of IBS symptoms and school performance were evaluated by a questionnaire. Demographic information such as age, gender, and body measurement was collected through medical record review. Vitamin D levels were measured by serum 25-hydroxyvitamin D. br＞ b＞Results/b＞: The average vitamin D level of IBS patients is 12.3±1.9ng/mL. This study shows that vitamin D level is significantly lower in winter than summer and autumn. There was no statistically significant difference in the average vitamin D level, as well as baseline demographic data, according to the types of IBS. Association of the severity of abdominal pain with vitamin D level shows negative correlation (p=0.001). Also, there was a statistically significant difference in the mean vitamin D level (p0.001) between the good and bad school performance groups. br＞ b＞Conclusion/b＞: This study shows that the average vitamin D level is low in IBS adolescents. There is no difference of the average vitamin D level according to the types of IBS. Based on the result of this study, the severity of abdominal pain and school performance in IBS patients are related to vitamin D level. Therefore, vitamin D supplement could be considered as a choice of therapeutic methods to Korean IBS adolescent patients. /p＞