We report a case of an 18-year-old female who presented with severe hy
pomagnesemia due to idiopathic renal magnesium wasting. Only a partial
response was noted with magnesium oxide replacement. Treatment with t
he potassium-sparing diuretic amiloride led to a further rise in the s
erum magnesium level, a decrease in the fractional excretion of magnes
ium and improvement in her symptoms. This case demonstrates an effecti
ve treatment for idiopathic renal magnesium wasting.