Periodic breast cancer screening with mammography and clinical examination is being evaluated to determine its value in reducing breast cancer mortality among women. Representative samples of women aged 40 to 64 years enrolled in the Health Insurance Plan of Greater New York are randomly assigned to study and control groups, each of which will contain 30,000 women. Results of the study to date are consistent with the hypothesis that the screening leads to earlier detection of breast cancers than is ordinarily experienced and that mammography contributes significantly to detection. While these relationships are encouraging, they must be viewed with caution since the study is still in its early stages. Furthermore, the crucial question is whether mortality from breast cancer is lowered because of the screening, and definitive findings on this issue will require at least five years of follow-up.