mmune thrombocytopenia (ITP) is an acquired autoimmune disorder characterized by isolated thrombocytopenia defined as platelet count in peripheral blood <100 x 109 /L. ITP manifests with sudden purpura and hematoma in previously healthy children. Skin changes such as petechiae and purpura can be found in 60% of children, whereas mucous membrane and organ bleeding can be found in 40% of children. Vitamin D (VD) is a pro-hormone produced from 7-dehydrocholesterol when being exposed to radiation of ultraviolet light. The active form of VD affects the absorbance of calcium and phosphate in the intestine, calcium mobilization in bones and reabsorbance of calcium in the kidneys. In recent years the importance of VD has been examined in immune and inflammatory response. VD is also the regulator of the hematopoietic system, it modulates activation and proliferation of lymphocytes, induces differentiation of monocytes and inhibits secretion of several cytokines in T-cells. It has been demonstrated that supplementation with VD is beneficial for Th1- mediated autoimmune response, and that VD insufficiency turns immune response towards loss of tolerance. Scientists have found connection between hypovitaminosis D and higher incidence of autoimmune diseases, including autoimmune cytopenias. Aim of this study was to find correlation between values of 25-hydroxyvitamin D (25(OH)D) and severity of clinical presentation of newly diagnosed ITP using IBLS and SMOG bleeding scores. We also analyzed correlation between 25(OH)D values and platelet count. As our secondary outcome we assessed the correlation between the patient gender and age with SMOG score value, and also correlation of the same variables with 25(OH)D values. Influence of body mass on the VD values and clinical severity of ITP we analyzed with a correlation between body mass index (BMI) z score and 25(OH)D values. Medical records of 60 pediatric patients with newly diagnosed immune thrombocytopenia in the coastal region of Croatia were evaluated. The severity of bleeding was assessed using two bleeding scores, ITP Bleeding Scale (IBLS) and Skin-Mucosa-Organ-Gradation (SMOG). We found statistically significant negative correlation between 25(OH)D values and SMOG score (p=0,002), and between 25(OH)D values and IBLS score (p=0,048). Children with lower 25(OH)D values had higher values of the SMOG and IBLS scores and respectively more severe bleeding at the diagnosis of ITP. With further analysis of main domains of SMOG score, we found that patients with lower 25(OH)D value had more severe bleeding in the skin (p<0,0001) and organs (p=0,01). We did not find any statistically significant correlation 58 between 25(OH)D value and platelet count. However, there was statistically significant negative correlation between 25(OH)D value and patients age (p=0,001), as well as statistically significant positive correlation between SMOG score and patients age (p=0,02). We did not find statistically significant correlation between 25(OH)D value and BMI z score. In conclusion, our findings suggest that hypovitaminosis D influences the severity of the clinical presentation of ITP in children at the initial diagnosis of the disease. Therefore, therapy with VD could be a new potential option for treatment of ITP but for that further studies are needed.