Background Mortality and morbidity of acute myocardial infarction remains high. Intravenous magnesium started early after the onset of myocardial infarction is a promising adjunctive treatment that may limit infarct size, prevent serious arrhythmias, and reduce mortality. Several earlier trials and meta-analyses demonstrated a mortality rate reduction with magnesium treatment, but one mega trial found no benefit. Objective To examine the effect of intravenous magnesium versus control on early mortality and morbidity, stratified by time since onset of symptoms (lt;6 hours, 6+ hours), use of thrombolysis (used, not used), dose of magnesium used (lt;75 mmol, 75+ mmol). Search strategy We search the Cochrane controlled trial register (CCTR) of Cochrane Library, Medline and Embase. We also search Chinese Biomedical Disk (CBM disk) to identify the Chinese trials. Each database will be searched from its starting date to the first-half year of 2002. Selection criteria All randomized controlled trials that compared intravenous magnesium with placebo in the presence or absence of fibrolytic therapy in addition to routine treatment are eligible if they reported mortality and clinical events within 35 days of onset, regardless of language. Methods of review A data abstraction form will be specifically developed to extract information from the eligible articles. The quality assessment of RCT will be focused on method of treatment assignment, blinding of participants and investigators, control of selection bias after treatment assignment. The selection of studies, data extraction and assessment of methodological quality will be performed independently by two reviewers. Disagreements will be resolved through discussion, when necessary, in consultation with a third reviewer. Publication bias, heterogeneity and sensitivity analysis will be performed. The odds ratio (OR) will be used to pooling the effect if appropriate.