The profile of electrophysiologic effects of magnesium on the heart suggests that magnesium might be effective in the treatment of atrial fibrillation (AF) in terms of rhythm and rate control. We aimed to investigate the efficacy of magnesium administration in the acute treatment of rapid AF. Randomized controlled trials comparing intravenous magnesium versus placebo or antiarrhythmic agents for the acute management of rapid AF were included. Nine electronic databases were searched for relevant trials from the earliest possible dates through June 2005, as were abstract books from 8 cardiovascular meetings held in the past 10 years. We analyzed all outcomes using a fixed-effect model because of the low number of trials in each comparison. The results were expressed as relative risks (RRs) or odds ratios (ORs) for dichotomous outcomes and weighted mean differences for continuous outcomes, along with their 95% confidence intervals (CIs). Data were pooled for 4 trials (n=303) and 8 trials (n=476), respectively, for rate control (<100 beats/min) and rhythm control. Magnesium was effective in achieving rate control (OR 1.96, 95% CI 1.24 to 3.08) or rhythm control (OR, 1.60, 95% CI 1.07 to 2.39). An overall response was achieved in 86% and 56% of patients in the magnesium and control groups, respectively (OR 4.61 95% CI 2.67 to 7.96). Time to response (in hours) was significantly shorter in the magnesium group (weighted mean difference, -6.98; 95% CI -9.27 to -4.68). The risk of having a major adverse effect in the magnesium group was similar to that in the placebo group (RR 0.85, 95% CI 0.44 to 1.61). In conclusion, the present meta-analysis of published data suggests that intravenous magnesium administration is an effective and safe strategy for the acute management of rapid AF.