Hypomagnesemia and sub-clinical atherosclerosis are common in type 1 diabetic patients, and are especially common in poorly controlled and chronically treated diabetics. The aim of this study was to evaluate the relationships between serum magnesium (Mg) and intima-media thickness (IMT), and functions of common carotid artery (CCA), accepted as markers of early carotid atherosclerosis in type 1 diabetic patients. Serum magnesium levels were measured in 51 diabetic patients (26 girls and 25 boys). Age, duration of diabetes as well as major cardiovascular risk factors, including anthropometric and metabolic parameters were matched between girls and boys. CCA structure and functions were measured by ultrasonography as IMT, cross-sectional compliance (CSC), cross-sectional distensibility (CSD), diastolic wall stress (DWS), and incremental elastic modulus (IEM). The difference for serum Mg was significant between diabetic girls and boys (0.75+/-0.09 mmol/l versus 0.86+/-0.14 mmol/l; p=0.02). Mg levels were correlated with IMT (r=-0.58, p=0.004) and DWS (r=-0.49, p=0.01) in girls and with IMT (r=-0.55, p=0.007) and DWS (r=-0.46, p=0.03) in boys. In a multivariate regression model, Mg, systolic blood pressure and waist-hip ratio emerged as independent correlates for mean IMT in girls with the total variance explained being 63%, and Mg and triglycerides emerged as independent correlates for mean IMT in boys with the total variance explained being 68%. Mg did not emerge as independent correlates for mean DWS in both girls and boys. Serum magnesium levels in children and adolescent patients with type 1 diabetes are associated with early atherosclerosis.