From a report in The New York Times:
Regular use of low-dose aspirin does not prevent first heart attacks in women younger than 65, as it does in men, a 10-year study of healthy women has found.
The participants in the Women’s Health Study who took 100 milligrams of aspirin every other day were no less likely to suffer heart attacks than the participants in another group who took placebos. Each group had about 20,000 members.
But aspirin did appear to help protect the women against one kind of stroke – something the drug has not been found conclusively to do for men.