Hypomagnesemia is the commonest electrolyte abnormality in the ambulatory diabetic patient and is also a frequent finding in patients with diabetic ketoacidosis. Excessive urinary magnesium loss associated with glycosuria is probably the most important factor in the genesis of hypomagnesemia in the diabetic patient. The clinical consequences of magnesium deficiency include impairment of insulin secretion, insulin resistance and increased macrovascular risk. The role of magnesium deficiency in microvascular complications has yet to be clearly defined.