The data from population-based, randomized trials of the mammographic screening of women aged 40 to 49 years are limited by retrospective subgroup analysis, low statistical power, and the use of older mammographic techniques. Nonetheless, meta-analysis involving the most recently reported data from all similarly designed trials shows a 21% mortality reduction in women who have had mammographic screening compared with those in control groups, a statistically significant difference (upper bound of 95% CI, 0.98). Evidence from modern mammography demonstration projects (which are limited by a lack of control groups and the unavailability of mortality results) also shows that the tumor size, axillary lymph node status, and stage of screening-detected breast cancers in women aged 40 to 49 years indicate that screening is at least as beneficial for these women as it is for those aged 50 to 64 years, for whom mammographic screening is widely accepted. Overall, the evidence suggests that screening will benefit women aged 40 to 49 years, but some argue that the level of proof provided is not rigorous enough to establish the efficacy of screening. In the United States, more than 10 000 deaths per year occur among women who develop breast cancer between the ages of 40 and 49. Thousands of lives are probably lost each year because women are not being screened. We believe that it is much more prudent to endorse mammographic screening now, risking the unlikely subsequent determination that the effort was ineffective, than to withhold screening until it is determined whether “proof” of efficacy will be obtained, risking the loss of so many lives. We urge primary care practitioners to recommend routine mammography for women aged 40 to 49 years.