Background Higher serum levels of magnesium (Mg(2+)) may contribute to improved outcome following ischemic stroke, and this may be related to vessel recanalization. Patients with low or normal serum magnesium levels during the acute phase of ischemic stroke may be more susceptible to neurologic deterioration (ND) and worse outcomes. Methods All patients who presented to our center within 48 hours of acute ischemic stroke (July 2008 to December 2010) were retrospectively identified. Patient demographics, laboratory values, and multiple outcome measures, including ND, were compared across admission serum Mg(2+) groups and change in Mg(2+) from baseline to 24-hour groups. Results Three hundred thirteen patients met inclusion criteria (mean age: 64.8 years, 42.2% female, 64.0% black). Mg(2+) groups at baseline were not predictive of poor functional outcome, death, or discharge disposition. Patients whose serum Mg(2+) decreased during the first 24 hours of admission were also not at greater odds of ND or poor outcome measures compared with patients with unchanging or increasing Mg(2+) levels. Conclusions Our results suggest that patients who have low Mg(2+) at baseline or a reduction in Mg(2+) 24 hours after admission are not at a higher risk of experiencing ND or poor short-term outcome. Ongoing prospective interventional trials will determine if hyperacute aggressive magnesium replacement affords neuroprotection in stroke. © 2013 Elsevier B.V. All rights reserved.