The clinical course of 2 patients with primary hypomagnesemia is reported. In one male patient, 5 months old, measurements of magnesium retention, intestinal absorption, fecal excretion and renal clearance were performed. The retention (2.8%) and absorption (7.8%) of 28-Mg were markedly reduced in comparsion to controls (average retention 25% and average absorption 28%). The retention values of the parents and other realtives did not differ from those of healthy adults. The examined patient was sucessfully treated with trimagnesium dicitrate containing 1.75 g magnesium per day.