Magnesium is primarily (99%) an intracellular cation. In contrast to calcium, the maintenance of magnesium homeostasis is highly dependent on dietary intake, and there is no known regulatory system that functions to mobilize magnesium from bone or elsewhere to maintain circulating extracellular levels.1Magnesium is involved in maintaining the ionic cellular balance, eg, by its role in the function of the cell membrane sodium-potassium adenosine triphosphatase pump.2 Magnesium is an obligate ion essential for the activation of > 300 enzymes,3 for virtually all hormonal reactions occurring in the body, and for the activity of adenylate cyclase.1 Finally, magnesium also acts as a calcium channel blocker.4Magnesium thus undoubtedly is a major player in many cellular and hormonal functions. And severe magnesium deficiency is dangerous: in critically ill patients, for instance, hypomagnesemia occurs in up to 65% of patients, and is associated with increased mortality rates.6 Severe magnesium deficiency can lead, among other things, to a variety of dysrhythmias, seizures, muscle weakness, and mental status changes, various endocrine dysfunctions, but also to bronchospasm and respiratory failure.1 Magnesium replacement hence undoubtedly is useful in these critically ill patients.3 But is it useful for John Doe’s asthma? To answer this question, it may be useful to apply Koch’s postulates: (1) Is magnesium a bronchodilator? (2) Is asthma characterized by/associated with magnesium deficiency states? (3) Is magnesium therapy useful in treating asthma?