Despite the publication of several randomized controlled studies, there is still much debate on whether magnesium sulfate improves outcome in patients with aneurysmal subarachnoid hemorrhage. Here we present data to assess the clinical effectiveness of magnesium sulfate in the prevention of cerebral vasospasm in patients who have suffered from aneurysmal subarachnoid hemorrhage. The EMBASE and PubMed databases were searched using the following terms: "magnesium sulfate" or "MgSO(4)" with "subarachnoid hemorrhage" or "cerebral vasospasm". A manual search of the bibliographies of relevant articles was also conducted. Two co-authors of the present study designed the meta analysis of published randomized clinical trials and extracted the data. Data were analysed by using Review Manager 4.2 from the Cochrane Collaboration (Oxford, UK). Five published manuscripts were identified according to the screening criteria. The occurrence of poor outcome (death, vegetative state, or dependency) in patients treated with magnesium sulfate was less likely than control group patients (odds ratio [OR] 0.54 [95% confidence interval, CI 0.36-0.81]). Mortality rates did not differ between magnesium sulfate (14%) and control treated (12%) patients (OR 1.16 [95% CI 0.51-2.65]). Our results indicate that although there was reduced likelihood of a poor outcome for patients treated with magnesium sulfate after SAH, patient mortality was not improved.