Many studies have pointed out that patients with vitamin D deficiency have a longer stay in the intensive care unit and a poor prognosis. The investigators' previous multi-center prospective observational study in Taiwan reveals that the prevalence of vitamin D deficiency in critically ill patients in northern Taiwan is 59%, and the prevalence of severe vitamin D deficiency is 18%. The investigators used the data of that observational study to train a predictive model for predicting vitamin D deficiency. In addition, the association between vitamin D and the immune regulation of critically ill patients in Taiwan has not been investigated. This study aims primarily to validate the performance of the prediction model of vitamin D deficiency. Moreover, this study will investigate the association between vitamin D level and inflammatory cytokine levels. This multi-center prospective observational study will enroll critically ill patients admitted to intensive care units (ICUs) less than 28 days. After inform consent, blood will be drawn for examination of vitamin D, interleukin 6, and interleukin 10 levels. The main diagnosis of ICU admission, past medical history, vital signs within 24 hours of admission, disease severity, and laboratory data will be recorded. The predictive model will use the required parameters to predict the patient's risk of vitamin D deficiency and vitamin D severe deficiency.