The aim of our study was the determination of fractional excretion of magnesium (FEMg++) in both normal subjects and hypomagnesemic patients. 142 subjects aged 26-72 years, recruited from our lipid clinic (control population) and 74 hypomagnesemic patients, aged 36-75 years, were studied. The mean FEMg++ in the control population was 1.8 per cent (range, 0.5-4 per cent). FEMg++ was not correlated with either serum magnesium or with serum creatinine. The mean FEMg++ in patients with hypomagnesemia of extrarenal origin was 1.4 per cent (range, 0.5-2.7 per cent), while the mean FEMg++ in hypomagnesemic patients in whom renal magnesium loss was the main etiologic factor for the pathogenesis of hypomagnesemia was 15 per cent (range, 4-48 per cent). In both groups of hypomagnesemic patients FEMg++ was positively correlated with the urinary magnesium to creatinine molar ratio, but was not correlated with serum magnesium or creatinine levels. FEMg++ could better distinguish the two groups of hypomagnesemic patients than the urinary magnesium to creatinine molar ratio. Hypomagnesemic patients exhibited a cluster of other acid-base and electrolyte abnormalities, mainly respiratory alkalosis, hypokalemia, hypophosphatemia, and hypocalcemia. In conclusion, in hypomagnesemic patients with normal renal function FEMg++ is a very useful tool for the diagnostic approach of hypomagnesemia. A value more than 4 per cent is indicative of inappropriate magnesium loss.