Eclampsia, the occurrence of a seizure in association with pre-eclampsia, remains an important cause of maternal mortality. Although it is standard practice to use an anticonvulsant for management of eclampsia, the choice of agent is controversial and there has been little properly controlled evidence to support any of the options. 1687 women with eclampsia were recruited into an international multicentre randomised trial comparing standard anticonvulsant regimens. Primary measures of outcome were recurrence of convulsions and maternal death. Data are available for 1680 (99.6%) women: 453 allocated magnesium sulphate versus 452 allocated diazepam, and 388 allocated magnesium sulphate versus 387 allocated phenytoin. Most women (99%) received the anticonvulsant that they had been allocated. Women allocated magnesium sulphate had a 52% lower risk of recurrent convulsions (95% CI 64% to 37% reduction) than those allocated diazepam (60 [13.2%] vs 126 [27.9%]; ie, 14.7 [SD 2.6] fewer women with recurrent convulsions per 100 women; 2p < 0.00001). Maternal mortality was non-significantly lower among women allocated magnesium sulphate. There were no significant differences in other measures of serious maternal morbidity, or in perinatal morbidity or mortality. Women allocated magnesium sulphate had a 67% lower risk of recurrent convulsions (95% CI 79% to 47% reduction) than those allocated phenytoin (22 [5.7%] vs 66 [17.1%] ie, 11.4 [SD 2.2] fewer women with recurrent convulsions per 100 women; 2p < 0.00001). Maternal mortality was nonsignificantly lower among women allocated magnesium sulphate. Women allocated magnesium sulphate were also less likely to be ventilated, to develop pneumonia, and to be admitted to intensive care facilities than those allocated phenytoin. The babies of women who had been allocated magnesium sulphate before delivery were significantly less likely to be intubated at the place of delivery, and to be admitted to a special care nursery, than the babies of mothers who had been allocated phenytoin. There is now compelling evidence in favour of magnesium sulphate, rather than diazepam or phenytoin, for the treatment of eclampsia.