Objective: Serum Leptin concentration reflects the body fat mass. There is controversial reports about the insulin effect on serum Leptin concentration. We wanted to examine the effect of insulin therapy on serum Leptin in children with new onset type I diabetes.

Materials and Methods: This was a Cross-Sectional study in Children's Medical Center on 34 children who had new onset type I diabetes. Serum Leptin level was measured at presentation, as a baseline before initiation of insulin therapy, three to five days and after three months of beginning of insulin therapy. The linear regression by SPSSv.16 used for analysis.

Results: There was a meaningful difference between the baseline level of Leptin and 3th-5th day. (P=0.00). The dosage of insulin was the most important factor affecting the Leptin levels after three months of treatment (P=0.006). In third months of follow up, sex and insulin dosage were the variables that effected on Leptin level. (P=0.003).

Conclusion: The results of our study showed that children with new-onset type I diabetes have low Leptin level before insulin therapy which increased in 3-5 days after treatment. We concluded that the acute insulin therapy alters the Leptin secretion/ action.