Will screening mammography save my life?. The answer depends on the reference class. The life-saving absolute benefit of mammography increases as the absolute death risk in the woman's prospective screening subset increases. For women starting at age 50 and assuming a 20% relative risk reduction (RRR), the chance that "mammography saves lives" appears as the bottom row for each of the five screening subsets. For instance, mammography saves 4.3% of screen-detectable cancer patients' lives (subset B). These life-saving percentages correspond to the RRR (A: Breast cancer deaths), life-saving proportions (B&C), reduction in absolute death risk (D: Repeated screening mammography – see Figure 1), and average benefit (E: Single mammogram). For a 20% RRR, the underlying absolute death risk (middle row) is five times the life-saving percentage. The number of events needed to save one life in each subset is the reciprocal of the life-saving percentage and increases from the smallest subset A (5) to the largest subset E (2970). Potential screening harm (top row, 100% minus bottom row) for women with cancer includes overdiagnosis, overtreatment, and delayed diagnosis. The potential harm for healthy women includes false-positive evaluations and biopsies, screening associated anxiety, and radiation-induced cancer.