Objective: Magnesium is involved in about 600 enzymatic and metabolic reactions. In patients at risk, e.g. elderly, a magnesium deficiency has to be taken into account. This can result in a variety of diseases, e.g. cardiovascular disorder and hypertension. Diagnosis can be made by laboratory, anamnesis (drugs) or clinical symptoms. Especially elderly (up to 30 %) or hypertensives are risk patients for a magnesium deficiency. In these patients a magnesium supplementation is then often of benefit. Design and method: In our study 45 elderly hypertensives with normal renal function were investigated (serum creatinine was 1.1+/-0.1 mg/dL). All patients were older than 60 years. None of our patients took magnesium supplements. All patients had an essential hypertension grade 1 and 2 according to ESH guidelines classification. Anti-hypertensive treatment was performed using usual drugs (e.g. betablockers, calcium antagonists or Renin-Angiotensin-Aldosterone System (RAAS) blockers), no diuretics for at least 4 weeks before start of the study. Serum magnesium concentrations were measured by a Cobas Apparatus, Roche, Germany. The analysis of the measurements of ionized magnesium concentrations was carried out with a Prime Plus system from NOVA Medical Devices Corp., Rödermark, Germany. This is a point-of-care unit device with a high precision standard. The tests use either capillary blood or about 1 mL of whole blood and the complete measurement process takes about only 1 minute. Results: In serum magnesium content only 1 patient was below normal range of 1.7 md/dl. Mean+/- s.d. was 1.96+/- 0.16 mg/dl. In contrast in 12 of 45 patients ionized magnesium was significantly below the normal range (27%, 0.54+/-0.06 mmol/l). There was no correlation between serum or ionized magnesium concentrations (n.s., Pearson’ s or Spesrmann rank test, p value 0.91 and 0.41). Bood pressure was controlled. Conclusions: In conclusion normal serum magnesium values do not exclude a magnesium deficiency in elderly hypertensive patients. The determination of ionized magnesium might be a better tool in this context. A magnesium deficiency is often observed in elderly and has to be substituted in a sufficient amount to avoid cardiovascular and other diseases.