It has been known for long that renal Mg excretion is increased in patients with type I diabetes mellitus, and that these patients have a Mg deficit. It can be hypothesized, that this deficit might be related to the development of late complications in the diabetic. In recent years it has been shown that the increased renal Mg excretion in patients with type I diabetes is due primarily to an elevated plasma glucose concentration. An increase in plasma glucose concentration from 5 to 12 mmol/l more than doubles renal Mg excretion, if everything else is kept constant. Hyperinsulinism may also contribute to the increased renal Mg excretion. However, since improved metabolic control in patients with type I diabetes reduces the renal Mg loss despite an increase in insulin dosage, hyperinsulinism is probably of minor importance in the aetiology of hypermagnesuria in patients with type I diabetes mellitus, compared with the effect of hyperglyeaemia.