Despite major advances in intensive care, sepsis continues to be a major cause of morbidity and mortality. Vitamin D is involved in various physiologic functions, including cellular responses during infection and inflammation. The aim of this study was to evaluate diagnostic value of 25-hydroxyvitamin D in childhood sepsis because it can be fatal if diagnosis delayed. The study included 40 children with sepsis and 20 children without sepsis (control group). We included only the patients with high probable sepsis, judged by clinical and laboratory findings, including positive blood culture. Blood samples were collected from patients with sepsis before treatment (pre-treatment group) and 48-72 hours later (post-treatment group). Treatment varied from ampicillin-sulbactam to cephalosporin. Blood samples were collected from control group once on admission. Serum 25-hydroxyvitamin D levels were significantly higher in sepsis (pre-treatment group) than control group (74 +/- 8 ng/ml vs. 28 +/- 12 ng/ml, p = 0.01) and the serum 25-hydroxyvitamin D levels were decreased to 44 +/- 5 ng/ml (p = 0.01) after treatment. Moreover, we found significant positive correlation between 25-hydroxyvitamin D and each of well-know sepsis markers, C-reactive protein, tumor necrosis factor-a and interleukin-6. A cut-off point of 20 ng/mL for serum 25-hydroxyvitamin D showed 84% sensitivity and 76% specificity for sepsis diagnosis. This is the first study evaluating the diagnostic role of vitamin D in pediatric sepsis, thereby suggesting that serum 25-hydroxyvitamin D level can be used as a diagnostic marker for sepsis with high sensitivity and specificity.