We describe a patient with the eosinophilia-myalgia syndrome (EMS) with persistent myalgias, cramping, and weakness that were not responsive to treatment. Despite a normal serum magnesium level, a loading study was performed, and the results suggested low tissue levels of magnesium. He was given parenteral magnesium and had dramatic improvement in symptoms as well as in muscle intracellular levels of this cation. After cessation of magnesium therapy the symptoms recurred, and magnesium repletion again led to an improvement in symptoms and ATP levels. Low tissue levels of magnesium, even in the setting of normal serum levels, may lead to the neuromuscular symptoms in EMS and related disorders.