Magnesium (Mg) has important effects within the vascular system. Magnesium deficiency was shown to trigger vasoconstriction and enhance vascular endothelial injury, thus promoting the development and progression of atherosclerosis. However, it is still not completely understood whether low serum Mg also promotes the occurrence of stroke. We hereby intended to investigate Mg levels in serum in the early stage of ischemic stroke and to evaluate the relationship between serum Mg concentration and the development of neurological deficits. The study included forty patients with acute ischemic stroke (26 women and 14 men), mean age 56 +/- 4 years, without any other serious injuries. Twenty-one healthy subjects, sex- and age-matched were selected as controls. The serum Mg concentrations were measured colorimetrically on a Hitachi 917 autoanalyzer. Serum levels of Mg were checked on admission, and at 48 hours after the onset of ischemic stroke. Using NIHSS, the neurological deficit was assessed on the 1st day, and 48 hours later. Statistical analysis was performed using the Student t test. The results confirm that there is a relationship between a low Mg concentration in serum at 48 hours after onset of ischemic stroke and the intensity of the neurological deficit. Mean value was 1.39 +/- 0.213 mmol/L (on admission), 1.47 +/- 0.181 mmol/L (at 48 hours after the onset of stroke) versus 1.66 +/- 0.138 mmol/L (in controls). Severity of paresis degree was higher in the patients with low Mg levels (p < 0.05). The serum Mg concentration has been suggested to possibly affect the neurologic state. A decrease in the serum Mg concentration indicates the severity of the injury. A magnesium substitution therapy may be useful.