Oral magnesium (Mg) supplementation can improve insulin sensitivity and secretion in patients with Type 2 diabetes mellitus (DM). We studied the effect of Mg supplementation on glycaemic control, blood pressure, and plasma lipids in insulin-requiring patients with Type 2 DM. Fifty moderately controlled patients were randomized to 15 mmol Mg or placebo daily for 3 months. Plasma Mg, glucose, HbA 1c , lipids, erythrocyte Mg, Mg and glucose concentrations in 24-h urine, and systolic and diastolic pressure were measured before and after 3 months treatment. Plasma Mg concentration was higher after supplementation than after placebo (0.82 Β± 0.07 vs 0.78 Β± 0.08 mmol l -1 , pΟ½0.05), as was Mg excretion (5.5 Β± 1.9 vs 3.7 Β± 1.4 mmol 24 h -1 , p = 0.004) but erythrocyte Mg concentrations were similar. No significant differences were found in glycaemic control (glucose: 10.7 Β± 3.8 vs 11.6 Β± 6.2 mmol l -1 , p = 0.8; HbA 1c : 8.9 Β± 1.6 vs 9.1 Β± 1.2%, p = 0.8), lipids or blood pressure. On-treatment analysis (34 patients: 18 on Mg, 16 on placebo) yielded similar results. An increase in plasma Mg concentration irrespective of medication was associated with a tendency to a decrease in diastolic pressure (increased plasma Mg vs no increase: -4.0 Β± 10.1 vs +2.5 Β± 12.0 mmHg, p = 0.059). Three months' oral Mg supplementation of insulin-requiring patients with Type 2 DM increased plasma Mg concentration and urinary Mg excretion but had no effect on glycaemic control or plasma lipid concentrations.