Magnesium is a predominantly intracellular ion, and it is a cofactor in more than 300 enzymatic reactions, like tyrosinokinase activity. Its deficiency may increase insulin resistance, especially in patients with metabolic syndrome or type 2 diabetes. This study evaluated in 27 patients with poorly controlled type 2 diabetes if there was correlation between intracellular magnesium levels, laboratorial indexes of insulin resistance and glycemic control. Decreased serum and intracellular magnesium depletion were found in 75% and 30.8% of patients, respectively. A negative correlation between intracellular magnesium levels (ICMg) and BMI and HbA1 was found. The homeostasis model assessment for insulin resistance (HOMA-IR) was higher than 3.0 in 59.2% of patients and there was a tendency to negative correlation with ICMg levels, although without statistical significance. Despite the small number of patients, this study shows that magnesium deficiency is frequent in patients with diabetes and its correlation with insulin resistance should be more studied.