To test the blood pressure (BP)-lowering effect of oral magnesium supplementation (that is, magnesium chloride (MgCl(2)) solution) in diabetic hypertensive adults with hypomagnesaemia not on diuretic treatment but receiving concurrent captopril, we conducted a double-blind, placebo-controlled trial. Eighty-two subjects between 40 and 75 years of age were randomly enrolled. Over 4 months, subjects in the intervention group received 2.5 g of MgCl(2) (50 ml of a solution containing 50 g of MgCl(2) per 1000 ml of solution) equivalent to 450 mg of elemental magnesium, and control subjects inert placebo. The primary trial end point was a reduction in systolic (SBP) and diastolic (DBP) blood pressure. Complete follow-up was achieved for 79 of the 82 randomized subjects. SBP (-20.4+/-15.9 versus -4.7 +/- 12.7 mm Hg, P=0.03) and DBP (-8.7+/-16.3 versus -1.2+/-12.6 mm Hg, P=0.02) showed significant decreases, and high-density lipoprotein-cholesterol (0.1+/-0.6 versus -0.1+/-0.7 mmol l(-1), P=0.04) a significant increase in the magnesium group compared to the placebo group. The adjusted odds ratio between serum magnesium and BP was 2.8 (95%CI: 1.4-6.9). Oral magnesium supplementation with MgCl(2) significantly reduces SBP and DBP in diabetic hypertensive adults with hypomagnesaemia.