Background: Extracellular magnesium (Mg) accounts for approximately 1% of the total body Mg. Clinically, serum Mg concentration is measured, but it does not necessarily reflect total body Mg status. Although relationships have been reported between reduced Mg and cardiovascular disease in non-dialysis patients, there have been few such studies in hemodialysis patients. It was hypothesized that reduced Mg, as represented by lower Mg concentration in the hair, would be associated with echocardiographic parameters in chronic hemodialysis patients. Methods and Results: Hair Mg concentration was measured in 79 male hemodialysis patients using inductively coupled plasma mass spectrometry, and the relationships between hair Mg concentration and echocardiographic parameters were investigated. There was no significant correlation between Mg concentration in the hair and in serum. Hair Mg concentration in the patients with high-left ventricular mass index (LVMI) was significantly lower than that in the low-LVMI patients. Hair Mg concentration correlated significantly and negatively with posterior left ventricular wall thickness, interventricular septum thickness, left ventricular wall thickness (LVWT), and relative wall thickness. Serum Mg concentration, however, did not correlate with any of these echocardiographic parameters. Conclusions: In hemodialysis patients, hair Mg concentration is a biomarker, independent of serum Mg concentration. Hair Mg, but not serum Mg, was significantly and negatively associated with LVWT. Reduced tissue Mg concentration, as measured in the hair, may be associated with left ventricular hypertrophy in hemodialysis patients.