Magnesium is the second more abundant intracellular cation of the human body after potassium, being essential in great number of enzymatic and metabolic processes. Is a cofactor of all the enzymatic reactions that involve the ATP and comprises of the membrane that maintains the electrical excitability of the muscular and nervous cells. A low magnesium level is bound in malabsorption syndrom, diuretics or aminoglucoside therapy; hyperparathyroidism or diabetic acidosis. Elevated concentration of magnesium is bound in uremia, chronic renal failure, glomerulonephritis. Addison's disease or intensive anti acid therapy 1, 4, 5. Clinical diagnosis should not be made on a single test results; it should integrate clinical and other laboratory data.