Ageing constitutes a risk factor for magnesium deficit. Primary magnesium deficit originates from two aetiological mechanisms: deficiency and depletion. Primary magnesium deficiency is due to insufficient magnesium intake. Dietary amounts of magnesium are marginal in the whole population whatever the age. Nutritional deficiencies are more pronounced in institutionalized than in free-living ageing groups. Primary magnesium depletion is due to dysregulation of factors controlling magnesium status: intestinal magnesium hypoabsorption, reduced magnesium bone uptake and mobilization, sometimes urinary leakage, hyperadrenoglucocorticism by decreased adaptability to stress, insulin resistance and adrenergic hyporeceptivity. Secondary magnesium deficit in ageing largely results from various pathologies and treatments common to elderly persons, i.e., non-insulin dependent diabetes mellitus and use of hypermagnesuric diuretics. Magnesium deficit may participate in the clinical pattern of ageing, particularly in neuromuscular, cardiovascular and renal symptomatologies. The consequences of hyperadrenoglucocorticism-the simplest marker of which is non-response to the dexamethasone suppression test-may include immunosuppression, muscle atrophy, centralization of fat mass, osteoporosis, hyperglycaemia, hyperlipidaemia, atherosclerosis, and disturbances of mood and mental performance through accelerated hippocampal ageing particularly. It seems very important to point out that magnesium deficit and stress aggravate each other in a true 'pathogenic vicious circle', particularly in the stressful state of ageing. The importance of magnesium deficit in the aetiologies of insulin resistance, and the adrenergic, osseous, oncogenic, immune and oxidant disturbances of ageing is still uncertain. Oral physiological magnesium supplementation (5 mg Mg/kg/d) is the best diagnostic tool for establishing the importance of magnesium deficiency. Too few open and double blind studies on the effects of the treatment of magnesium deficiency and of magnesium depletion in geriatric populations have been done. Further study is necessary to assess the true place of magnesium deficit in the pathophysiology of ageing.