To evaluate the effectiveness of magnesium sulphate in the treatment of eclampsia and pre-eclampsia by a systematic quantitative overview of controlled clinical trials. Online searching of the MEDLINE database between 1966 and 1995, and scanning of the bibliography of known primary studies and review articles on the use of magnesium sulphate in eclampsia and pre-eclampsia. Study-selection, study quality assessment and data extraction were performed independently by two reviewers under masked conditions. Where possible outcome data from trials were pooled and summarised using the Mantel-Haenszel method. One thousand seven hundred and forty-three women with eclampsia and 2390 with pre-eclampsia included in nine randomised trials that evaluated the effects of magnesium sulphate. Seizure activity and maternal death. In eclampsia, recurrence of seizures was less common with magnesium sulphate therapy compared with phenytoin (odds ratio [OR] 0.27, 95% CI 0.17-0.45, P = 0.00) and diazepam (OR 0.41, 95% CI 0.30-0.57, P = 0.00). As indicated by the point estimate, there was a trend towards a reduction in maternal mortality with magnesium sulphate in eclampsia (OR 0.51, 95% CI 0.24-1.07, P = 0.10 versus phenytoin; OR 0.78, 95% CI 0.41-1.45, P = 0.52 versus diazepam). When used for seizure prophylaxis in pre-eclampsia, magnesium sulphate was found to be more effective than phenytoin (OR 0.15, 95% CI 0.03-0.72, P = 0.01). Magnesium sulphate is a superior drug in preventing the recurrence of seizures in eclampsia and in seizure prophylaxis in pre-eclampsia.