Modern screen-film mammography with molybdenum-anode X rays results in tissue doses being delivered primarily by photons with an energy of less than 20 keV. Such photons interact with tissue predominantly through the photoelectric effect, producing low-energy electrons that have different patterns of energy deposition at the cellular level compared with those from higher-energy X rays. These differences result in low doses of typical molybdenum-based mammography X rays having an estimated radiobiological effectiveness of approximately 1.3 compared with 80 kVp or 250 kVp X rays, and approximately 2 compared with higher-energy gamma rays. Thus the risk from mammography could be higher, by such factors, than previously estimated. This would result in the optimal age for beginning mammographic screening, derived from risk-benefit ratios, being increased by at least 1-2 years and possibly by as many as 10 years.