Cathelicidins are antimicrobial peptides that play a critical role in innate immune defense against invasive bacterial infections. Several studies have suggested the potential role of vitamin D in maintaining immune homeostasis and preventing the development of autoimmune processes e.g., diabetes mellitus. The aim of the present work was to study serum level of vitamin D and cathelicidin in a group of Egyptian children with type 1 diabetes mellitus and to know their relation to infection in those cases. It was conducted at Alzahraa University hospital on sixty children, aged 3-15 years of both sexes: forty of them known to have type 1 diabetes mellitus (type 1 DM) as defined by ADA, 2012 (twenty with acute bacterial infection and twenty without infection), and twenty were apparently healthy children age and sex matched taken as a control group. Patients were taken randomly from outpatient clinic of National Diabetes Institute during the period from January 2013 to June 2013. All studied children were subjected to careful history taking, thorough clinical examination and laboratory investigations including: CBC, CRP, measurement of serum 1,25(OH)2 vitamin D and serum cathelicidin levels by ELISA. For diabetic children fasting blood sugar, postprandial blood sugar and glycated hemoglobin (HbA1c) were also measured. Results revealed a significantly higher mean serum cathelicidin level in diabetic with infection when compared to controls (38.57±33.6 vs 18.67±25.07) while no significant difference was found between either diabetic without infection and controls or between diabetic with and without infection. The mean serum vitamin D levels in the diabetic patients (both with and without infection) were significantly lower than its mean serum level in controls (23.89±7.39 pg/ml and 24.49±5.37 versus 31.30±6.78 gp/ml respectively). Also, a significant positive correlation was found between serum cathelicidin and BMI while there was no significant correlation between serum cathelicidin or vitamin D and other studied parameters. In conclusion,serum cathelicidin level was significantly higher in diabetic children with acute bacterial infection when compared to controls, all studied diabetic children had significant lower mean serum vitamin D level in comparison to controls and a significant positive correlation was found between serum cathelicidin and BMI.