BACKGROUND: The influence of long-term use of aspirin on total mortality in women remains uncertain. METHODS: We conducted a prospective, nested, case-control study of 79 439 women enrolled in the Nurses' Health Study who had no history of cardiovascular disease or cancer. Women provided data on medication use biennially since 1980. We assessed relative risk (RR) of death according to aspirin use before diagnosis of incident cardiovascular disease or cancer and during the corresponding period for each control subject. RESULTS: During 24 years, we documented 9477 deaths from all causes. In women who reported current aspirin use, the multivariate RR of death from all causes was 0.75 (95% confidence interval, 0.71-0.81) compared with women who never used aspirin regularly. The risk reduction was more apparent for death from cardiovascular disease (RR, 0.62; 95% confidence interval, 0.55-0.71) than for death from cancer (RR, 0.88; 95% confidence interval, 0.81-0.96). Use of aspirin for 1 to 5 years was associated with significant reductions in cardiovascular mortality (RR, 0.75; 95% confidence interval, 0.61-0.92). In contrast, a significant reduction in risk of cancer deaths was not observed until after 10 years of aspirin use (P(linear trend) = .005). The benefit associated with aspirin was confined to low and moderate doses and was significantly greater in older participants (P(interaction)< .001) and those with more cardiac risk factors (P(interaction) = .02). CONCLUSIONS: In women, low to moderate doses of aspirin are associated with significantly lower risk of all-cause mortality, particularly in older women and those with cardiac risk factors. A significant benefit is evident within 5 years for cardiovascular disease, whereas a modest benefit for cancer is not apparent until after 10 years of use.