OBJECTIVE: To determine whether migraineurs may have a systemic deficiency of magnesium. BACKGROUND: Magnesium deficiency has been shown to play a potential role in the pathogenesis of migraine, but there are no data on total body magnesium status in migraineurs. METHODS: An oral magnesium load test was performed by giving 3000 mg of magnesium lactate during a 24-hour interictal period to 20 patients with migraine (15 women and 5 men; mean age, 37.9 years) and 20 healthy volunteers (16 women and 4 men; mean age, 39.6 years). Baseline and postload magnesium concentrations were determined from serum and 24-hour urine specimens. RESULTS: There was no significant difference between the groups in the baseline serum and urine magnesium concentrations, although the latter tended to be lower (P = .064) in the migraine group. The postload magnesium concentrations were significantly higher within both the migraine (P < .0001 and P < .0001) and the control (P = .0009 and P < .0001) groups compared to the baseline values. After loading, the 24-hour urinary magnesium excretions were significantly lower (P = .0007) in the patients with migraine than in the controls, but serum values did not differ. CONCLUSIONS: Magnesium retention occurs in patients with migraine after oral loading, suggesting a systemic magnesium deficiency.