Although many theories and hypotheses have been offered for the etiology of tension-type headache (TH), no one previous hypothesis seems to adequately explain TH. This may, in large measure, account for why it is often difficult to effectively treat TH. Herein, we review current and old hypotheses of TH and offer a new hypothesis which is consistent with what is known about TH. We show that magnesium (Mg) metabolism may be pivotal in both the etiology and treatment of TH. Measurement of serum ionized Mg2+ (IMg2+) levels and brain intracellular free Mg2+ ([Mg2+]i) appear to offer excellent methods for establishing the validity of our hypothesis. Since approximately 70% of patients who have a TH exhibit muscular tightness and tenderness, it is distinctly possible that problems in Mg metabolism and dietary intake are the links to concomitant muscle tension and TH. The significance of release of pain mediators, muscle cramps, muscle strains (and damage) and muscle tension to TH, and its relationship to Mg metabolism, are reviewed. These are all associated with a Mg-deficient state. It seems clear from the available data that TH's are more associated with muscle tension or scalp tension than any other headache type. From the data available, Mg supplementation appears to be of great benefit in many of these situations. We believe there is a great need for clinicians to examine Mg2+ metabolism, bioavailable Mg2+ in muscle tissues and blood, and the effectiveness of Mg salts (in a double-blinded, placebo-controlled manner) in subjects with TH and muscle tension.