Deaths from heart disease have declined by 30 percent in older women who have taken estrogen for ten years or more, according to a study announced at the American Heart Association's annual epidemiology meeting Thursday. The research studied white women aged 65 to 74. No benefits were shown for women older than 75, or in those who took the drug for less than 10 years. Heart disease strikes six in nine American women and killed 237,000 American women in 1990. While other studies have indicated a beneficial link between estrogen and heart disease, researchers are not certain why the hormone seems to help.