BACKGROUND Reasons for persistent differences in breast cancer mortality rates among various racial and ethnic groups have been difficult to ascertain. OBJECTIVE To determine reasons for disparities in breast cancer outcomes across racial and ethnic groups. METHODS Prospective cohort. METHODS The authors pooled data from 7 mammography registries that participate in the National Cancer Institute-funded Breast Cancer Surveillance Consortium. Cancer diagnoses were ascertained through linkage with pathology databases; Surveillance, Epidemiology, and End Results programs; and state tumor registries. METHODS 1,010,515 women 40 years of age and older who had at least 1 mammogram between 1996 and 2002; 17,558 of these women had diagnosed breast cancer. METHODS Patterns of mammography and the probability of inadequate mammography screening were examined. The authors evaluated whether overall and advanced cancer rates were similar across racial and ethnic groups and whether these rates were affected by the use of mammography. RESULTS African-American, Hispanic, Asian, and Native American women were more likely than white women to have received inadequate mammographic screening (relative risk, 1.2 [95% CI, 1.2 to 1.2], 1.3 [CI, 1.2 to 1.3], 1.4 [CI, 1.3 to 1.4], and 1.2 [CI, 1.1 to 1.2] respectively). African-American women were more likely than white, Asian, and Native American women to have large, advanced-stage, high-grade, and lymph node-positive tumors of the breast. The observed differences in advanced cancer rates between African American and white women were attenuated or eliminated after the cohort was stratified by screening history. Among women who were previously screened at intervals of 4 to 41 months, African-American women were no more likely to have large, advanced-stage tumors or lymph node involvement than white women with the same screening history. African-American women had higher rates of high-grade tumors than white women regardless of screening history. The lower rates of advanced cancer among Asian and Native American women persisted when the cohort was stratified by mammography history. CONCLUSIONS Results are based on a cohort of women who had received mammographic evaluations. CONCLUSIONS African-American women are less likely to receive adequate mammographic screening than white women, which may explain the higher prevalence of advanced breast tumors among African-American women. Tumor characteristics may also contribute to differences in cancer outcomes because African-American women have higher-grade tumors than white women regardless of screening. These results suggest that adherence to recommended mammography screening intervals may reduce breast cancer mortality rates.