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Background: Older black women are less likely to undergo mammography and are more often given a diagnosis of advanced-stage breast cancer than older white women., Objective: To investigate the extent to which previous mammography explains observed differences in cancer stage at diagnosis between older black and white women with breast cancer., Design: Retrospective cohort study using the Linked Medicare-Tumor Registry Database., Setting: Population-based data from three geographic areas of the United States included in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program (Connecticut; metropolitan Atlanta, Georgia; and Seattle-Puget Sound, Washington)., Participants: Black and white women 67 years of age and older in whom breast cancer was diagnosed between 1987 and 1989., Measurements: Medicare claims were used to classify women according to mammography use in the 2 years before diagnosis as nonusers (no previous mammography), regular users (> or =2 mammographies done at least 10 months apart), or peri-diagnosis users (mammography done only within 3 months before diagnosis). Information on mammography use was linked with SEER data to determine cancer stage at diagnosis. Stage was classified as early (in situ or local) or late (regional or distant)., Results: Black women were more likely to not undergo mammography (odds ratio [OR], 3.00 [95% CI, 2.41 to 3.75]) and to be given a diagnosis of late-stage disease (OR, 2.49 [CI, 1.59 to 3.92]) than white women. When women were stratified by previous mammography use, the black-white difference in cancer stage occurred only among nonusers (adjusted OR, 2.54 [CI, 1.37 to 4.71]). Among regular users, cancer was diagnosed in black and white women at similar stages (adjusted OR, 1.34 [CI, 0.40 to 4.51]). In logistic modeling, previous mammography alone explained about 30% of the excess late-stage disease in black women. In a separate model, previous mammography explained 12% of the excess late-stage disease among black women after adjustment for sociodemographic and comorbidity information., Conclusion: Differences in breast cancer stage at diagnosis between older black and white women are related to previous mammography use. Increased regular use of mammography may result in a shift toward earlier-stage disease at diagnosis and narrow the observed differences in stage at diagnosis between older black and white women.