BACKGROUND: Anticonvulsants are used for pre-eclampsia in the belief they prevent eclamptic convulsions, and so improve outcome. Evidence supported magnesium sulphate as the drug to evaluate. Methods Eligible women (n=10 141) had not given birth or were 24 h or less postpartum; blood pressure of 140/90 mm Hg or more, and proteinuria of 1+ (30 mg/dL) or more; and there was clinical uncertainty about magnesium sulphate. Women were randomised in 33 countries to either magnesium sulphate (n=5071) or placebo (n=5070). Primary outcomes were eclampsia and, for women randomised before delivery, death of the baby. Follow up was until discharge from hospital after delivery. Analyses were by intention to treat.FINDINGS: Follow-up data were available for 10 110 (99•7%) women, 9992 (99%) of whom received the allocated treatment. 1201 of 4999 (24%) women given magnesium sulphate reported side-effects versus 228 of 4993 (5%) given placebo. Women allocated magnesium sulphate had a 58% lower risk of eclampsia (95% CI 40—71) than those allocated placebo (40, 0•8%, vs 96, 1•9%; 11 fewer women with eclampsia per 1000 women). Maternal mortality was also lower among women allocated magnesium sulphate (relative risk 0•55, 0•26—1•14). For women randomised before delivery, there was no clear difference in the risk of the baby dying (576, 12•7%, vs 558, 12•4%; relative risk 1•02, 99% CI 0•92—1•14). The only notable difference in maternal or neonatal morbidity was for placental abruption (relative risk 0•67, 99% CI 0•45—0•89).Interpretation: Magnesium sulphate halves the risk of eclampsia, and probably reduces the risk of maternal death. There do not appear to be substantive harmful effects to mother or baby in the short term.Indexed in MEDLINE