The investigators are conducting a prospective cross-over study to evaluate the effects of vitamin D supplementation on diabetes control and the pro-inflammatory markers involved in microvascular complications in adolescents with Type 1 Diabetes. The investigators expect to see a significant improvement in glycemic control and a reduction of serum pro-inflammatory markers in adolescents with Type 1 Diabetes and vitamin D deficiency or insufficiency, who are treated with vitamin D.

Inclusion Criteria:

1. Between 13 to 21 years of age, with at least Tanner stage 4 sexual maturity for males or post-menarchal females, and T1DM for at least 1 year. To ensure that inclusion criteria for sexual maturity are met, a physical exam for research purposes will be performed.
2. HbA1c between 7 to 9%
3. Adequate renal function (serum creatinine < 1.5 mg/dL in males and < 1.2 mg/dL in females) and adequate liver function (AST and ALT < 2.5 times the upper limit of normal)
4. Vitamin D insufficiency or deficiency (25-OH vit D level < 30ng/mL) which will be determined on initial screening labs after consenting subjects.

Exclusion Criteria:

1. Less than 13 or greater than 21 years of age
2. Less than Tanner stage 4 sexual maturity for males or pre-menarche
3. HbA1c less than 7% or greater than 9%
4. T1DM for less than 1 year
5. Vitamin D sufficient (25-OH vit D level > 30 ng/mL)
6. Currently taking any medication that can interfere with vitamin D synthesis or metabolism, including but not limited to Orlistat, Phenobarbital, Dilantin, Anti-tuberculosis drugs
7. Currently taking any medication other than insulin that alters blood glucose levels, including but not limited to systemic glucocorticoids
8. Inadequate renal function (serum creatinine > 1.5mg/dL in males and > 1.2mg/dL in females) or inadequate liver function (AST and ALT > 2.5 times the upper limit of normal)
9. Evidence of malabsorption or short gut.