Abstract
The impact of mammography screening upon the chances of a woman dying from breast cancer has been studied for four decades. Until this century the results have always been presented in terms of benefit to populations, and have erroneously been assumed to be directly transferable to individual women. A multitude of factors combine to reduce the effect of screening upon a population as measurable in a randomized controlled trial. The most serious problem has been the misunderstanding that an invitation to mammography screening would be equivalent to actually getting a mammogram. Additionally, the control population also benefits from the screening program, and these benefits reduce the measurable effect of screening. Recent long-term trials, which have been able to fully document participation in mammography screening on an individual basis, have demonstrated an even more substantial reduction in breast cancer mortality attributable to mammography screening, with a much more limited impact of therapeutic advances against advanced breast cancer.