Although mammography was introduced in the United States in the late 1970s, Medicare did not cover screening mammography until 1991.1 The delay reflected a broader historical reluctance to cover preventive services. The Social Security Act of 1965, which established Medicare, initially excluded coverage of preventive services in part because these services were ill defined and of uncertain benefit at the time. This was also true of mammography. By 1991, to our knowledge, no clinical trials of mammography had included women older than 74 years, and there was skepticism about its effectiveness in this age group.1