Nebulized Magnesium Sulfate has been shown to be of benefit in adults with acute asthma exacerbations, though limited work has been done in the pediatric population. Current therapeutic questions include the effect of more than one dose of nebulized magnesium, the possibility of a sustained effect (greater than 20 minutes) after a treatment, the potential benefit in children younger than 5 years of age, and the use of an asthma score to re-assess patients after treatment with this medication. The purpose of this double-blind randomized placebo-controlled clinical trial is the evaluate the effect of multiple doses of nebulized magnesium sulfate versus saline in addition to standard asthma therapy on clinical asthma score and FEV1 in children 7 to 18 years of age with a moderate to severe acute asthma exacerbation. Our hypothesis is that nebulized magnesium sulfate, when added to traditional bronchodilator therapy, will improve acute asthma exacerbations more rapidly than standard therapy alone. The primary outcomes are asthma score and FEV1 values obtained after two nebulized magnesium sulfate compared to placebo treatments. This therapy will be supplemental to standard therapy of albuterol and ipratropium nebulized treatments and systemic corticosteroids. The secondary outcome is patient disposition (discharge home or hospitalization). Patients with known asthma will be approached for enrollment and informed consent obtained if the asthma score after the first albuterol treatment is "2" or greater. One hundred-seventy patients will be enrolled and randomized to either the treatment group or the placebo group. The change from baseline in asthma scores and FEV1 values will be compared among the control and treatment groups to assess for any benefit of the addition of nebulized magnesium sulfate to the treatment regimen.