Magnesium (Mg) status has previously not been properly assessed among healthy elderly subjects.Thirty-six healthy elderly subjects participated. Their Mg status was assessed by serum Mg, basal urinary Mg output, and with a Mg loading test (30 mmol infused during 8 hours; urine sampled 24 hours), and compared with 53 healthy younger subjects. Their dietary intake was assessed by a quantified food frequency questionnaire. Fourteen of the subjects received 300 mmol Mg to study the effect on Mg status.With the exception of vitamin D in women, average energy and nutrient intakes were adequate. All subjects had serum Mg levels within the reference value of the laboratory. Basal urinary Mg excretion was 3.3 +/−1.1 mmol/day and 24-hour Mg retention after a Mg load was 28 +/−16% compared to 6 +/−11% in younger controls, suggesting Mg deficiency in the elderly. In the 14 subjects who received oral Mg supplementation there was a statistically significant increase in basal urinary Mg excretion and creatinine clearance, and decreases in Mg retention, serum Mg and serum creatinine.This study suggests that a significant subclinical Mg deficit, not detected by serum Mg, was present in many of these healthy elderly subjects. Mg supplementation improved Mg status and renal function.