Objectives: Magnesium deficiency has been implicated as a factor in numerous chronic diseases and previous studies suggest a greater prevalence of occult magnesium deficiency among older adults. Serum is the choice for the assessment of most analyses used in clinical medicine, although serum magnesium concentrations have been shown to be poor predictors of intracellular magnesium concentration. The aim of this study was to compare intracellular and extracellular magnesium concentrations in geriatric outpatients. Moreover, we examined whether a significant correlation between magnesium parameters and clinical outcome existed. Design: Cross-sectional study. Setting: Geriatric medicine outpatient clinic of a university hospital. Participants: A total of 246 patients with a mean age of 71.9 +/- 5.7 years were involved. Measurements: Intra-erythrocyte magnesium levels were analyzed with atomic absorption spectrophotometry. Results: Serum magnesium levels were within normal range in all patients, whereas intra-erythrocyte magnesium measurements were low in 57% of the patients. Increase in serum levels were together with just only a slightly increase in intra-erythrocyte measurements and the relationship was very weak. Intra-erythrocyte Mg levels were not significantly correlated with many laboratory or clinical parameters. Conclusion: Our results confirm that intra-erythrocyte Mg does not correlate with serum levels and clinical parameters in geriatric outpatients, but further studies are needed to define the correlation.