Assessment of the effect of breast-cancer screening has been hampered by difficulty in measuring secular trends. In this study, data from a cancer registry were used to determine secular trends and to evaluate the effect of screening on breast-cancer mortality. On the basis of several randomized clinical trials, 1 – 3 the World Health Organization concluded in 2002 that screening mammography for women between the ages of 50 and 69 years reduced the rate of death from breast cancer by 25%. 4 Nevertheless, the use of screening mammography is still debated, chiefly because of concern regarding methodologic limitations in some of the randomized trials. 5 In addition, the benefit of mammography when implemented in a population-based service program remains poorly quantified. Therefore, continued evaluation of breast-cancer screening programs is warranted. 6 The main challenge in quantifying the reduction in mortality from nonrandomized screening programs is . . .