Magnesium (Mg) deficiency can contribute to cardiac dysrhythmias and may predispose to ischemic heart disease. Most diuretic agents cause loss of Mg, but serum levels may be normal despite cellular depletion. We studied the clinical characteristics of hypertensive patients treated for at least 6 months with either hydrochlorothiazide (HCTZ) or a single nondiuretic drug. To evaluate Mg status in our patients, we measured the percentage retention of a parenterally administered Mg load as an accurate indicator of functionally available total body Mg. Serum lipid, blood chemistry, and serum Mg values were obtained, and cardiac exercise testing and Holter monitoring were done. Levels of potassium were lower, but those of serum Mg were higher with HCTZ treatment despite double the Mg load retention. The Mg load retention data indicate relative Mg depletion in the HCTZ-treated group. Eighty percent of all patients studied had abnormal retention of Mg, even though their serum levels were normal. A percentage Mg load retention determination is needed to assess accurately Mg status.