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Summary Background Results of observational studies suggest that hormone replacement therapy (HRT) could reduce the risk of coronary heart disease (CHD), but those of randomised trials do not indicate a lower risk in women who use oestrogen plus progestagen. The aim of this study was to ascertain whether or not unopposed oestrogen reduces the risk of further cardiac events in postmenopausal women who survive a first myocardial infarction. Methods The study was a randomised, blinded, placebo controlled, secondary prevention trial of postmenopausal women, age 50–69 years (n=1017) who had survived a first myocardial infarction. Individuals were recruited from 35 hospitals in England and Wales. Women received either one tablet of oestradiol valerate (2 mg; n=513) or placebo (n=504), daily for 2 years. Primary outcomes were reinfarction or cardiac death, and all-cause mortality. Analyses were by intention-to-treat. Secondary outcomes were uterine bleeding, endometrial cancer, stroke or other embolic events, and fractures. Findings Frequency of reinfarction or cardiac death did not differ between treatment groups at 24 months (rate ratio 0·99, 95% CI 0·70–1·41, p=0·97). Similarly, the reduction in all-cause mortality between those who took oestrogen and those on placebo was not significant (0·79, 0·50–1·27, p=0·34). The relative risk of any death (0·56, 0·23–1·33) and cardiac death (0·33, 0·11–1·01) was lowest at 3 months post-recruitment. Interpretation Oestradiol valerate does not reduce the overall risk of further cardiac events in postmenopausal women who have survived a myocardial infarction.