We evaluated the effects of oral magnesium supplementation on plasma lipid concentrations in patients with non-insulin-dependent diabetes mellitus. Twenty-six persons with non-insulin-dependent diabetes mellitus received 4.5 g magnesium pidolate/d for one month, and 17 persons received placebo. Before and at the end of the treatment period cholesterol concentration, triglycerides, LDL cholesterol, HDL cholesterol, and plasma and erythrocyte magnesium were evaluated. Chronic magnesium supplementation produced a significant reduction of plasma cholesterol and LDL cholesterol, and an increase of HDL cholesterol. These results suggest that oral supplementation of magnesium may be useful in the treatment of hyperlipidaemia in patients with non-insulin-dependent diabetes mellitus.