In the last few years, it has been suggested that magnesium plays a central role in establishing a threshold for migraine attacks and in intervening with the pathogenetic mechanisms involved in their onset. Accordingly, we determined red blood cell magnesium levels in adult migraine patients with and without aura interictally and in some of them also ictally. In comparison with normal subjects, migraineurs with and without aura had significantly lower red blood cell magnesium levels in the interictal period. Ictal red blood cell magnesium levels did not differ from the interictal values. Low red blood cell magnesium levels could be a peripheral expression of the reduced brain magnesium concentration observed in migraine patients.