Hypomagnesemia in childhood is relatively frequently noted in the neonatal period due to maternal causes, such as decreased intake due to vomiting, overuse of laxatives, and neonatal causes such as intrauterine growth retardation, birth asphyxia and exchange transfusion. A very rare cause of neonatal magnesium deficiency is called primary hypomagnesemia caused by impaired intestinal absorption of magnesium. Reference values of serum magnesium in cord blood are slightly lowered. Erythrocyte magnesium content is also lowered in cord blood and during the first month after birth. Mononuclear magnesium content shows no differences with age. Renal magnesium loss is diagnosed by the presence of hypomagnesemia with an inappropriately high 24-hour urinary magnesium excretion. In isolated familial hypomagnesemia an autosomal dominant as well as an autosomal recessive mode of inheritance was found. The renal magnesium threshold is lowered in both forms but the tubular maximum is only lowered in the dominant form. In familial hypomagnesemia-hypokalemia (Gitelman syndrome) the renal magnesium threshold is lowered but the tubular maximum is in the normal range. In this syndrome, with probably an autosomal recessive mode of inheritance, the renal defect might be located in the distal nephron after the medullary part of the ascending limb of the loop of Henle. The magnesium content of mononuclear cells and erythrocytes is in the normal and lower normal range, respectively. In the familial hypomagnesemia-hypercalciuria syndrome, hypomagnesemia is always combined with hyperuricemia and nephrocalcinosis. Myopia and horizontal nystagmus are often present.