The objective of our study is to quantitatively examine the available evidence regarding the efficacy and side effects of magnesium sulfate for acute tocolysis (from randomized trials) compared with placebo and beta- agonist agents. Randomized trials comparing magnesium sulfate with placebo or beta-agonists for tocolysis were identified with a MEDLINE-based search and was supplemented by a search of obstetrical textbooks and bibliographies. Trials underwent quality evaluation and data abstraction by two independent, blinded investigators. Outcomes evaluated included delivery delay of various durations as well as the frequency of major and minor side effects. Summary odds ratios and 95 percent confidence intervals for dichotomous outcomes were calculated using a random effects model. Interstudy heterogeneity for these outcomes was assessed with a Q statistic. We identified 12 randomized controlled trials of magnesium sulfate for acute tocolysis. Four studies were excluded because of either lack of comparison of magnesium sulfate to either placebo or beta-agonists or lack of reporting clinical outcomes of interest. The eight remaining randomized trials comparing magnesium sulfate with placebo or beta-agonists were included in this analysis. There was no significant difference between MgSO4 and placebo for any of the measured outcomes for delay in delivery. Comparing magnesium sulfate to ritodrine or beta-agonists did not demonstrate any differences between the agents in achieving clinically significant tocolysis. There was a significant difference between MgSO4 and beta-agonists in the frequency of medication discontinuation because of side effects, but not in the frequency of major adverse drug events. There are few data comparing magnesium sulfate with a placebo for acute tocolysis. Magnesium sulfate seems to be comparable to ritodrine and beta-agonists, although the available data are not sufficient for a rational choice between these agents.