6559 Background: Delays in follow-up after abnormal mammograms can lead to worse outcomes and may contribute to health disparities. BIRADS 0 mammograms necessitate additional diagnostic imaging, and BIRADS 4 or 5 mammograms should be followed by biopsy. The goal of this study was to investigate racial disparities in rates and timeliness of (1) diagnostic follow-up after a BIRADS 0 screening mammogram and (2) biopsy following a subsequent BIRADS 4 or 5 diagnostic mammogram. Methods: We included women ≥18 years old who underwent a screening mammogram at the Hospital of the University of Pennsylvania with an assessment of BIRADS 0 between September 2010 and February 2018. The distributions of time from screening to diagnostic mammogram and from BIRADS 4 or 5 diagnostic mammogram to biopsy were estimated using the Kaplan Meier method. Follow-up was censored at 365 days. Case-mix adjusted Cox proportional hazards models were used to estimate the association between race/ethnicity and time to diagnostic mammogram and biopsy. Results: We identified 6299 women (Asian/PI=257, Black=3223, Hispanic=124, White=2420, Other/Unknown=275) with 6880 BIRADS 0 screening mammograms during the study period. Following these BIRADS 0 mammograms, the overall rate of diagnostic mammograms within 365 days was 87.3% (n=6006 mammograms), with a rate of 90.6% (2432) for White women and 85.3% (2971) for Black women. For the 1151 BIRADS 4-5 diagnostic mammograms in the cohort, the overall rate of follow-up biopsies within 365 days was 91.8% (n=1057 biopsies), with a rate of 93.8% (396) for White women and 91.1% (575) for Black women. Compared to mammograms obtained by White women, those obtained by Black women were less likely to be followed up with a diagnostic mammogram (HR 0.71, 95% CI 0.63-0.80, p<0.001) and biopsy (HR 0.74, 95% CI 0.55-0.98, p=0.037) when indicated (Table). Almost 1/4 (24.2%, 95% CI 23.1-25.9%) of BIRADS 0 screening mammograms among Black women were not followed by diagnostic imaging within 30 days as compared to 14.6% among White women (95% CI 13.3-16.0%, p<0.001). 23.6% (95% CI 20.5-27.2%) of BIRADS 4-5 diagnostic mammograms among Black women were not followed up with biopsy within 30 days vs 18.7% for White women (95% CI 15.4-22.8%, p=0.61) (Table). These disparities persisted at 90 days. Conclusions: Racial disparities exist in rates of follow-up after BIRADS 0 mammograms. The additive effects of delays at each diagnostic step put Black women at disproportionately greater risk for worse outcomes. [Table: see text]