Three children with familial hypomagnesemia from infancy were treated perorally with magnesium for 9 to 12 years. Their somatic and intellectual development have since been normal. Without therapy, the serum magnesium fell from subnormal (about 0.5 mmoles/liter) to very low values (0.2 to 0.3 mmoles/liter) within 1 to 4 wk. We observed a secondary fall in serum calcium and potassium and an increase in sodium and phosphate although serum concentrations of PTH, calcitonin, and 25-OH-vitamin D in the blood remained normal. Balance studies confirmed the presence of a defect in the intestinal absorption of magnesium and excluded a defective renal tubular transport system. The subjects continued to require daily magnesium supplements to avoid serious symptoms. Optimal dosage was found to be in the range 0.5 to 0.75 mmoles/kg . day; doses above this caused diarrhoea and a fall in the serum and urine levels of magnesium. Pathophysiologic mechanisms involved in the electrolyte changes that occurred secondarily to the hypomagnesemia are discussed.