Objectives. We assessed whether population rates of mammography screening, and their changes over time, were associated with improvements in breast cancer stage at diagnosis and whether the strength of this association varied by race/ethnicity. Methods. We analyzed state cancer registry data linked to socioeconomic characteristics of patients’ areas of residence for 1990–1998 time trends in the likelihood of early stage diagnosis. We appended each cancer registry record with matching subgroup estimates of self-reported mammography screening. Results. Trends in screening and stage at diagnosis were consistent within groups, but African American women had a significantly lower proportion of early stage cancers despite an advantage in screening. Population screening rates were significantly associated with early diagnosis, with a weaker association in African American women than White women (odds ratio [OR] = 1.70; P<.0001 vs OR=2.02; P<.0001, respectively). Conclusions. Improvements in screening rates during the 1990s across racial/ethnic groups appear to have contributed significantly to earlier diagnosis within each group, but a smaller effect in African American women should raise concerns. A key health policy challenge is to ensure that screening effectively translates into earlier diagnosis.