A female Thai baby born to non-consanguineous parents, presented with primary hypomagnesemia at 10 weeks of age, and suffered recurrent convulsions that responded to magnesium supplementation. She was found to have hypomagnesemia (Mg 0.35-1.02 mEq/L) and a low urinary magnesium excretion of less than 10 mg per day, or urinary Mg/Cr that ranged from 0.005-0.01 mg/mg. Intermittent hypomagnesemia and one episode of hypocalcemia with occasional convulsions developed, due to irregular consumption of oral magnesium sulfate, which had a bitter taste, caused frequent loose stools and black staining of the teeth. Better compliance after switching from magnesium sulfate to magnesium oxide resulted in an increased level of serum magnesium and the gradual disappearance of the black staining of the teeth and frequent loose stools. The patient required an oral elemental magnesium dosage of 15-30 mg/kg/day to maintain the serum magnesium level at between 1.02-1.33 mEq/L and keep, her free from convulsions. The follow-up period was 7 years during which the patient showed normal physical growth and a mild degree of mental retardation.