Sixty children with acute moderate to severe asthma having an age range from 6-13 years attending the emergency room in the New Children’s Hospital were the subjects of this study. They were classified into two groups:GroupI (Study group) in which thirty patients suffering from acute exacerbation of asthma requiring hospitalization who received 3 B2 adrenergic nebulizer treatments, O2 and 5 mg/kg hydrocortisone, with no objective measures of improvement observed, then received I.V. magnesium sulfate 25 mg/kg (maximum 2 gm) in 100 ml saline.GroupI (Study group) in whichthirty patients suffering from acute exacerbation of asthma served as a control group. They received 3 B2 adrenergic nebulizer treatments, O2 and 5 mg/kg hydrocortisone, with no objective measures of improvement observed, then received 100 ml I.V. saline without magnesium (placebo).Both groups were assessed using serial vital signs and physical examination, peak expiratory flow rate (PEFR),PaO2 andserum magnesium concentration.This assessment was done at start of infusion, 10 minutes into the infusion , at completion of the infusion (20 minutes) then every 20 minutes 4 times up till 100 minutes post infusionComparison between group (I) before and after treatment showed:1- Two percent change in PO2 before and after treatment which is not clinically marked. 2- No significant change in PEFR before and after treatment.Comparison between group (II) before and after treatment showed:1- No Significant change in PO2 before and after treatment. 2- No Significant change in PEFR before and after treatment.And so from our results we can conclude that use of intravenous magnesium as an adjuvant therapy in children with acute severe asthma is not an effective method of treatment.