The purpose of this article was to review of the literature to determine whether MgSO4 provides an additive improvement in adults with acute asthma in the emergency department (ED). English-language published and unpublished studies (1968-1998) were retrieved using MEDLINE, SCIENCE CITATION INDEX, CURRENT CONTENTS, bibliographic reviews of primary research, review articles, consultation with experts, and the register of Medical Editors' Trial Amnesty. Meta-analysis of randomized, placebo controlled trials. The five selected trials included 374 patients with acute asthma (mean age 38.3 +/- 13.4 years, 30% men, 70% women). The main outcome measure was pulmonary function; admission rate was evaluated as a secondary goal. Individual and pooled effect sizes (ES) were determined for pulmonary function data. Binary outcome (admission rate) was combined and reported as odds ratio (OR). The overall summary ES (95% confidence interval [CI]) of the five trials satisfying all inclusion criteria, weighted by sample size was nonsignificant (ES = 0.02, -0.20 to 0.24, P = .84). This pooled ES was equivalent to 1.7%, 95% CI: -9.8 to 13.1%) greater in pulmonary function the MgSO4 compared with the control group. On the other hand, four studies addressed the effect of MgSO4 administration on hospital admissions. Pooled results revealed that MgSO4 did not decrease significantly admission rates (OR = 0.68, 95% CI: 0.41 to 1.15). The existing evidence reveals that the addition of MgSO4 to ED patients with moderate to severe asthmatic exacerbations does not alter treatment outcomes. Nevertheless, the number and size of studies being pooled remain small.