Objectives: This systematic review and meta-analysis was conducted to estimate the effects of intravenous and nebulized magnesium sulfate on treating adults and children with acute asthma. Methods: Electronic literature search and the manual search of key respiratory journals were performed up to October 18, 2011. Randomized controlled trials were included if patients had been treated with intravenous or nebulized magnesium sulfate in combination with beta 2-agonists and were compared with the use of beta 2-agonists. Standardized mean differences (SMDs) and the relative risks (RRs) were calculated for pulmonary functions and hospital admission respectively. Results: 25 trials (16 intravenous, 9 nebulized) involving 1754 patients were included. In adults intravenous treatment was associated with a significant effect upon respiratory function (SMD, 0.30; 95% confidence interval (CI), 0.05 to 0.55; p = 0.02) but weak evidence of effect upon hospital admission (RR 0.86, 95% CI 0.73 to 1.01; p = 0.06) in adults, and in children with significant effects upon both respiratory function (SMD, 1.94; 95% CI, 0.80 to 3.08; p = 0.0008) and hospital admission (RR, 0.70; 95% CI, 0.54 to 0.91; p = 0.008). Nebulized treatment was associated with significant effects upon respiratory function (SMD, 0.23; 95% CI, 0.06 to 0.41; p = 0.009) and hospital admission (RR, 0.63; 95% CI, 0.43 to 0.92; p = 0.02) in adults. Conclusion: The use of intravenous magnesium sulfate, in addition to beta 2-agonists and systemic steroids, in the treatment of acute asthma appears to produce benefits with respect to improve pulmonary function and reduce the number of hospital admissions for children, and only improve pulmonary function for adults. However, the use of nebulized magnesium sulfate just appears to produce benefits for adults. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.