Mammography is the only method of breast cancer screening that has established evidence of a mortality reduction. However, mammography does not achieve sufficient accuracy in the high-density breasts of patients <50 years of age. In 2009, the US Preventive Services Task Force revised its recommendation for breast cancer screening in women aged 40–49 years from Grade B to C because the net benefit was relatively small for this age bracket. The net benefit of screening is the sum of benefits and harm and should always be monitored especially in population screening. A high recall rate, an inefficient number needed to invite for screening to prevent one breast cancer death, a high false-positive rate and unnecessary additional imaging for women aged 40–49 years are great concerns of mammography screening. Overdiagnosis is also a detriment of mammography screening; however, it may have a limited effect on women aged 40–49 years. Establishment of new evidence for breast cancer screening, such as ultrasonography screening, is needed in order to create a more effective screening system.