In women older than 50 years, screening mammography offers the benefits of decreased mortality from breast cancer, increased use of conservative surgery, and the reassurance of being free of breast cancer after a negative examination. No similar data are available for younger women. The cost to avert a single death from breast cancer in younger women may be $2 million or more. One proposed strategy to improve the performance of screening mammography in younger women and to lower its cost is to restrict its use to women with risk factors for breast cancer, but this strategy will miss cases occurring in women who have no identifiable breast cancer risk factors. Because mammographic screening performance is different in younger women compared with older women, individual screening prescriptions based on risk may be appropriate until definitive trials demonstrate a mortality benefit in younger women. Additional research is needed to define the optimal screening strategy for both the entire population of women younger than 50 and those who are at increased risk for breast cancer.