Magnesium (Mg) deficiency seems to be implicated in immune dysfunction, including acute and chronic infections. Magnesium serum levels in 53 patients with acute ischemic stroke and acute bacterial (37 patients) and acute viral infections (16 patients) (29 men and 24 women, mean age 68.6+/-4.7 years) were analyzed as compared to 36 healthy subjects. As Mg is mainly an intracellular ion, assessment of Mg status is difficult. Mg serum levels were determined by the colorimetric method. Data were statistically analyzed. In patients with acute bacterial infections (sepsis, bronchopneumonia, urinary tract infections) a statistically significant decrease of Mg serum concentrations was found (1.26+/-0.12 mEq/L vs 1.69+/-0.14 mEq/L, p<0.001). In patients with acute viral infections, the decrease of Mg serum levels was significantly less (1.64+/-0.13 mEq/L vs 1.69+/-0.14 mEq/L, p<0.05). The change onset of Mg occurred within a few days and persisted for several weeks. These changes seemed to be non-specific and were independent of the agent causing bacterial infection. Patients with sepsis having a high degree of inflammation did not show a positive correlation between the severity of the disease and the changes of Mg. In conclusion, the measurement of Mg serum in bacterial infections is useful and physicians should maintain a high index of suspicion for hypomagnesemia and implement Mg therapy.