Although the United States had one of the world’s highest life expectancies during the first half of the 20th century, this survival advantage gradually eroded during the ensuing decades. Of particular concern in the context of this volume is the recent stagnation in mortality improvement among middle-aged and older U.S. women relative to both U.S. men and to women in other wealthy nations (Meslé and Vallin, 2006; Vaupel, 2003). These mortality patterns suggest an appealing but as yet unexplored explanation: the use of postmenopausal hormone therapy (HT) among women in the United States. There is considerable evidence that, at least prior to 2002, estrogen-type hormones had been widely prescribed to U.S. women after the cessation of menses not only for relief of unpleasant symptoms associated with menopause but also increasingly for presumed protection against cardiovascular disease and bone loss.1 National estimates suggest a steady increase in HT use since the early 1980s, with a prevalence of about 38 to 40 percent among women ages 50 to 74 in 1995. Data on numbers of prescriptions for all forms of HT reveal a continued increase from 1995 to 2001, with the annual number of prescriptions peaking at 92 million in 2000 (Hersh, Stefanick, and Stafford, 2004).