Vitamin D and its active form are potent immune-modulators and are known to play an important role in the glucose/insulin metabolism. Studies on type 2 diabetes mellitus (T2DM) have shown that vitamin D increases insulin efficacy; However, there are no studies that define a similar relationship in type 1 diabetes mellitus (T1DM). Aim of the work:To evaluate serum vitamin D status in children with type 1 diabetes mellitus with comparison to healthy subjects and to correlate its level with different clinical and laboratory parameters. Subjects and methods:This study included two groups; Group I: included 53 patients with T1D; 43 patients with poor glycemic control (group Ia ) and 10 patients with good glycemic control (group Ib). Another 15 apparently healthy children and adolescent's age and sex matched to the diseased group were taken as a control group and classified as group II. The studied groups were subjected to thorough history taking, clinical examination and laboratory investigations including: fasting blood glucose, glycated hemoglobin (HbA1c %), serum calcium level and 25-hydroxyvitD level. A serum 25OHD level of < 10 ng/mL was accepted as vitamin D deficiency, whereas < 20 ng/ mL was accepted as vitamin D insufficiency.Results: Serum 25-hydroxyvitD and serum calcium levels were significantly lower in T1D children than the control and in group Ia than group Ib. 25-hydroxyvitD had a significant strong positive correlation with serum calcium and significant strong negative correlations with fasting blood sugar and HbA1c %. Conclusion:T1D children had significantly lower serum 25-hydroxyl vit D and serum calcium levels than the control group. Vitamin D in diabetic persons is an important factor in glycemic control with subsequent prevention of further complications.