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A previous review by the Canadian Task Force on the Periodic Health Examination (now the Canadian Task Force on Preventive Health Care) in 1994 indicated fair evidence to exclude mammographic breast cancer screening of women aged 40–49 from the periodic health examination. This current review considers the available new and updated evidence regarding the effect of screening mammography on breast cancer mortality among women in this age group at average risk of breast cancer.Screening mammography starting at either age 40 or age 50.Reduction in breast cancer mortality.The MEDLINE and CANCERLIT databases were searched for relevant articles published from 1966 to January 2000. Of 68 references obtained, at least 22 were published after the 1994 review. To date, the only trial designed to assess the mortality benefits of screening mammography among women aged 40–49 did not have adequate power to exclude a clinically significant benefit. Other results from randomized controlled trials (RCTs) are post-hoc subgroup analyses of larger trials.Screening mammography offers the potential for significant benefits in addition to mortality reduction, including early diagnosis, less aggressive therapy and improved cosmetic results. However, the risks of screening include increased biopsy rates and the psychological effects of false reassurance or false-positive results. Although several of the trials reviewed constitute level I evidence (RCT), at present their conflicting results, methodologic differences and, most important, uncertainty about the risk:benefit ratio of screening precludes the assignment of a “good” or “fair” rating to recommendations drawn from them.The strength of evidence was evaluated using the methods of the Canadian Task Force on Preventive Health Care. A high value was placed on changes in survival. When evidence was available, value was also placed on potential quality-of-life implications.Current evidence regarding the effectiveness of screening mammography does not suggest the inclusion of the manoeuvre in, or its exclusion from, the periodic health examination of women aged 40–49 years at average risk of breast cancer (grade C recommendation). Upon reaching the age of 40, Canadian women should be informed of the potential benefits and risks of screening mammography and assisted in deciding at what age they wish to initiate the manoeuvre.The findings of this analysis were reviewed through an iterative process by the members of the Canadian Task Force on Preventive Health Care.The Canadian Task Force on Preventive Health Care is funded through a partnership between the Provincial and Territorial Ministries of Health and Health Canada.