Magnesium metabolism in 70 consecutive patients with renal stone disease was evaluated and compared with that of a control group matched for age and sex. Serum and urinary magnesium values were not different in the two groups, but magnesium output was greater in males than in females. Stone-formers excreted more calcium in their urine than did controls, and the urinary calcium and magnesium values for both groups were positively correlated. For both groups, the magnesium/calcium ratio in the urine declined with increasing calcium values but was lower in stone-formers than in controls for corresponding calcium values. Intestinal magnesium uptake was the same in both groups. Magnesium deficiency was not evident among stone-formers as assessed by the determination of the magnesium content in muscle specimens or by the retention of an intravenously administered magnesium load. Although stone-formers were not found to display any features of magnesium metabolism that were different from those in the control group, their lower urinary excretion of magnesium in relation to calcium may be a factor in their increased stone-forming propensity.