Objectives: Hypomagnesaemia and insulin resistance are two major clinical problems, with intertwining pathophysiology. We aimed to explore this association in obese patients and in non-insulin-treated patients with type 2 diabetes mellitus (T2DM).Methods: Subjects were recruited from the outpatient diabetes/obesity clinic of the Antwerp University Hospital. The population (N=2731) consists of 2 subject groups with different degrees of insulin resistance and insulin secretory potential: 1) overweight (Body Mass index (BMI) ≥ 25 kg/m² and <30 kg/m²) and obese (BMI ≥ 30 kg/m²) subjects, 2) adult T2DM patients. Hypomagnesaemia was defined as serum magnesium <1.7 mg/dl. Insulin resistance was estimated using the Homeostasis model assessment (HOMA-IR; cut-off point 2.82).Results: Hypomagnesaemia was present in 6.1% of the entire population. Patients with hypomagnesaemia had more visceral adipose tissue (VAT), and a higher HOMA-IR. They suffered more from the metabolic syndrome and T2DM. Patients with a HOMA-IR<2.82 were younger, had lower BMI and less VAT. They suffered less from hypomagnesaemia. Hypomagnesaemia was more prevalent in T2DM patients than in obese subjects without T2DM. Although serum magnesium and HOMA-IR were negatively correlated, logistic regression analysis showed that magnesium was not a significant predictor for HOMA-IR. Conclusions:Despite a significant negative correlation between magnesium and HOMA-IR, magnesium was not retained as a significant determinant of insulin resistance compared to the other predictors in our population of obese subjects and T2DM patients.