A goal of the Women's Health Study was to evaluate the balance of benefits and risks of low-dose aspirin in the primary prevention of stroke in healthy women. A total of 39,876 female health care professionals aged 45 years and older were randomly assigned to either low-dose aspirin (100 mg every other day) or placebo and followed for an average of 10 years. In the overall study population, aspirin significantly lowered the risk of total and ischemic stroke without affecting the risk of myocardial infarction or death from cardiovascular causes, leading to a nonsignificant finding with respect to the primary end point of total cardiovascular disease. In women aged 65 years or older, aspirin significantly reduced the risk of ischemic stroke, as well as myocardial infarction and total cardiovascular disease. While the findings suggest that aspirin should be considered for primary prevention of stroke in women aged 65 years or older, the balance of benefits and risks would not support recommending aspirin for all women in this age group. The discussion on whether to use aspirin should be had on an individual basis, assessing the net risks and benefits.