Background: Hypomagnesaemia is common in adult patients with both Type 1 Diabetes Mellitus (T1DM) and Type 2 Diabetes MellitusT2DM, especially in poorlycontrolled and chronically treated adults. However, to our knowledge, little research has addressed the relationship between serum magnesium levels and diabetes in children. The aim of the present study was to evaluate magnesium levels and examine their possible association with glycemic control in children with T1DM. Methods: In all, 150 children with T1DM aged between 2 and 16 years and 100 apparently healthy children aged between 2.5 and 16 years were recruited to the study. Using a cross-sectional design, we measured anthropometric parameters, HbA1c, serum magnesium, serum calcium, phosphorus, potassium, sodium, and urinary albumin (UA). Estimated glomerular filtration rate (eGFR), based on serum creatinine concentrations was also calculated. Results: Lower levels of magnesium were found in subjects with poor versus good glycemic control (0.81 ± 0.23 vs 0.96 ± 0.19 mmol/L, respectively; P = 0.001). Serum magnesium levels were negatively correlated with HbA1c (P = 0.002). Conclusions: The present study shows that serum magnesium levels were lowered in children and adolescents with T1DM and that low serum magnesium levels were associated with an increased risk of poor glycemic control, potentially contributing to the early development of diabetic complications.