BACKGROUND: In previous studies, unadjusted comparisons of mortality and major morbidity after acute myocardial infarction have generally indicated that women have a poorer outcome than men. Much larger studies are needed, with more complete adjustment for coexisting conditions, to determine whether this difference is explained by the older age of the women studied or by the presence of other unfavorable prognostic factors, or both. METHODS: As part of the Third International Study of Infarct Survival (ISIS-3), information was collected on deaths during days 0 to 35 and on major clinical events during hospitalization up to day 35 for 9600 women and 26,480 men with suspected acute myocardial infarction who were considered to have a clear indication for fibrinolytic therapy. We compared the outcome among women and men, first without adjustment, then with adjustment for age, and finally with adjustment for other recorded baseline characteristics by means of multiple logistic regression. RESULTS: The unadjusted odds ratio for death among women as compared with men was 1.73 (95 percent confidence interval, 1.61 to 1.86). The women were significantly older than the men, and after adjustment for age the odds ratio was reduced markedly to 1.20 (95 percent confidence interval, 1.11 to 1.29). Adjustment for other differences in base-line clinical features further reduced the odds ratio to 1.14 (95 percent confidence interval, 1.05 to 1.23). Excesses in other major clinical events among women were generally reduced to a similar extent by adjustment. CONCLUSIONS: It seems likely that there is at most only a small independent association between female sex and early mortality and morbidity after suspected acute myocardial infarction.