The recent reports from the Canadian National Breast Screening Study (CNBSS) address the effectiveness of breast cancer screening for women in the 40-49 age group and the benefit of adding mammography to standard clinical breast examination in women aged 50-59. Overall, the CNBSS results do not show reductions in breast cancer mortality after seven years of follow-up in either age group. The CNBSS is an important study that includes a large number of women, rigorous procedures, and thorough follow-up, but there are several caveats to interpreting these results. The study results reported to date are based on small numbers of end point events. There are questions about the effectiveness of the randomization procedures in creating cohorts that initially were at equal risk of breast cancer-related death. It also has been observed that the quality level of mammographic screens obtained at the beginning of the study were not as high as those obtained later. Future reports from the CNBSS are planned and may provide additional data helpful in interpreting the results.