Objective:To investigate the baseline levels of 25-hydroxy-vitamin D [25-(OH) D] in 3340 hospitalized children, analyze the 25-( OH) D levels in children with different diseases, age and gender in different seasons, and study the correlation between the 25-( OH) D levels and the clinical indicators. Methods:Totally 3340 hospitalized children were randomly selected, the 25-( OH) D levels were detected by an ELISA method, and Pearson correlation analysis of 25-( OH) D levels and clinical indicators such as liver function, myocardial enzymes, immunoglobulins, lymphocyte subtypes and thyroid function was performed. Results: The serum 25-(OH) D level in 3340 cases (1850 male, 1490 female) was (33. 00 ± 13. 42) ng·m1 -1, and that in those with neonatal diseases was the lowest followed by those with primary nephrotic syndrome, Henoch-schordeinpurpura and juvenile idiopathic arthritis. The ser-um 25-( OH) D levels in the premature was higher than that in full term infant. Except the newborn, the level of serum 25-( OH) D gradually reduced along with the age increase, while the percentage of insufficiency gradually increased. The serum 25-( OH) D level between the male and the female had no significant difference. The 25-( OH) D levels of hospitalized children were the highest in sum-mer. The serum level of 25-( OH) D was positively correlated with body mass index ( BMI) , alanine transaminase ( ALT) , aspartate transaminase (AST), creatine kinase (CK), creatine-phosphokinase (CK-MB), lactate dehydrogenase(LDH), free T4 (FT4) and free T3 (FT3), and negative correlation with alkaline phosphatase(ALP), immunoglobulin E (IgE) and immunoglobulin M (IgM). Conclusion:The incidence of vitamin D insufficiency is high in hospitalized children. The level of vitamin D is different among various diseases. The level of serum 25-( OH) D may have certain relevance with BMI, allergies, myocardial damage and thyroid function.