In this study we compared the content of muscle free [Mg2+] assessed by 31P MRS to that of serum total Mg assessed by routine colorimetric assays in 15 patients affected by Chronic Intestinal Failure (CIF) on long-term Home Parenteral Nutrition (HPN) or who had undergone isolated intestinal transplantation. We also investigated in the same cohort of patients the relationship of muscle free [Mg2+] and serum total Mg with parathyroid hormone (PTH) serum content. All patients showed a normal cytosolic free [Mg2+] in the calf muscle despite about half of them having a content of total serum [Mg] below or at the lower edge of the boundary limits. Muscle free Mg2+ and serum total Mg displayed an opposite correlation versus serum PTH, showing that the intracellular free form possesses a different functional metabolic meaning in the regulation of PTH secretion.