BACKGROUND: Randomized trials have shown that lowdose aspirin decreases the ri sk of a first myocardial infarction in men, with little effect on the risk of is chemic stroke. There are few similar data in women. METHODS: We randomly assigne d 39,876 initially healthy women 45 years of age or older to receive 100 mg of a spirin on alternate days or placebo and then monitored them for 10 years for a f irst major cardiovascular event (i.e., nonfatal myocardial infarction, nonfatal stroke, or death from cardiovascular causes). RESULTS: During followup, 477 majo r cardiovascular events were confirmed in the aspirin group, as compared with 52 2 in the placebo group, for a nonsignificant reduction in risk with aspirin of 9 percent (relative risk, 0.91; 95 percent confidence interval, 0.80 to 1.03; P=0 .13). With regard to individual end points, there was a 17 percent reduction in the risk of stroke in the aspirin group, as compared with the placebo group (rel ative risk, 0.83; 95 percent confidence interval, 0.69 to 0.99; P=0.04), owing t o a 24 percent reduction in the risk of ischemic stroke (relative risk, 0.76; 95 percent confidence interval, 0.63 to 0.93; P=0.009) and a nonsignificant increa se in the risk of hemorrhagic stroke (relative risk, 1.24; 95 percent confidence interval, 0.82 to 1.87; P=0.31). As compared with placebo, aspirin had no signi ficant effect on the risk of fatal or nonfatal myocardial infarction (relative r isk, 1.02; 95 percent confidence interval, 0.84 to 1.25; P=0.83) or death from c ardiovascular causes (relative risk, 0.95; 95 percent confidence interval, 0.74 to 1.22; P=0.68). Gastrointestinal bleeding requiring transfusion was more frequ ent in the aspirin group than in the placebo group (relative risk, 1.40; 95 perc ent confidence interval, 1.07 to 1.83; P=0.02). Subgroup analyses showed that as pirin significantly reduced the risk of major cardiovascular events, ischemic st roke, and myocardial infarction among women 65 years of age or older. CONCLUSION S: In this large, primary-prevention trial among women, aspirin lowered the ris k of stroke without affecting the risk of myocardial infarction or death from ca rdiovascular causes, leading to a nonsignificant finding with respect to the pri mary end point. BACKGROUND: Randomized trials have shown that low skies aspirin decrease the ri sk of a first myocardial infarction in men, with little effect on the risk of is chemic stroke. There are few similar data in women. METHODS: We randomly assigne d 39,876 initially healthy women 45 years of age or older to receive 100 mg of a spirin on alternate days or placebo and then monitored for 10 years for af irst major cardiovascular event (ie, nonfatal myocardial infarction, nonfatal stroke, or death from cardiovascular causes) During followup, 477 majo r cardiovascular events were confirmed in the aspirin group, as compared with 52 2 in the placebo group, for a nonsignificant reduction in risk with aspirin of 9 percent (relative risk, 0.91; 95 percent confidence interval, 0.80 to 1.03 ; P = 0 .13) With regard to individual end points, there was a 17 percent reduction in the risk of stroke in the aspirin group, as compared with the placebo group (rel ative risk, 0.83; 95 percent conf idence interval, 0.69 to 0.99; P = 0.04), due toa 24 percent reduction in the risk of ischemic stroke (relative risk, 0.76; 95 percent confidence interval, 0.63 to 0.93; P = 0.009) and a nonsignificant increa se in the risk of hemorrhagic stroke (relative risk, 1.24; 95 percent confidence interval, 0.82 to 1.87; P = 0.31). As compared with placebo, aspirin had no signi ficant effect on the risk of fatal or nonfatal myocardial infarction (relative r isk, 1.02; 95% confidence interval, 0.84 to 1.25; P = 0.83) or death from c ardiovascular causes (0.95; 95 percent confidence interval, 0.74 to 1.22; P = 0.68). Gastrointestinal bleeding requiring transfusion was more frequ ent in the aspirin group than in the placebo group (relative risk, 1.40; 95 perc ent confidence interval, 1.07 to 1.83; P = 0.02). Subgroup analyzes showed that as pirin significantly reduced the risk of major cardiovascular events, ischemic st roke, and myocardial infarction among women 65 years of age or older. CONCLUSION S: In this large, primary-prevention trial among women, aspirin lowered the ris k of stroke without affecting the risk of myocardial infarction or death from ca rdiovascular causes, leading to a nonsignificant finding with respect to the pri mary end point .