Background: Over 40% of all breast cancer (BC) is diagnosed in women age > 65y; majority will survive the initial diagnosis (dx), especially those with stage 0/1 disease. While disease biology contributes to the inferior outcome among non-Hispanic blacks (NHBs) observed in the early post-diagnosis phase, racial differences in latemortality – overall and cause-specific – and the contribution of comorbidity is not known. Methods: We used SEER data linked with Medicare claims to identify a cohort of 70,385 NHW and NHB BCS diagnosed with incident stage 0/1 disease between 2000 and 2011, at age > 65y, and surviving ≥ 2y from dx. Pre-cancer comorbidity was evaluated using Charlson Comorbidity Index (CCI), and pre-cancer depression, hypertension, and osteoporosis/fractures. Late mortality was defined as 5y survival conditional on surviving 2y after dx. Results: Median age at dx was 75y (66-105); 6.3% were NHB; CCI = 0: 66%, CCI = 2+: 12%. Compared to NHW, NHB BCS were younger at breast cancer dx ( < 75y: 56% vs. 51.5%, p < 0.01), had a higher level of poverty ( > 20% below poverty: 50.8% vs. 11.8%, p < 0.01), higher pre-cancer comorbidity (CCI = 2+ (24.3% vs. 10.7%, p < 0.01), and more often received no treatment for BC (4.5% vs. 2.1%, p < 0.01). 16,628 (23.6%) late deaths were observed (median follow-up: 7y [2-14]). Overall survival was 72.5% for entire cohort; NHW: 73% vs. NHB: 64.9%, p < 0.001; unadjusted HR = 1.4, p < 0.001). The racial difference in late mortality was mitigated after adjustment for age, treatment, poverty and pre-cancer comorbidity (HR = 1.13, p < 0.001). Cause-specific late mortality at 5y was higher in NHBs vs. NHWs for the most common causes of death (primary cancer: 8.2% vs. 4.1%, p < 0.001; CVD: 11.6% vs. 8.9%, p = 0.01; second cancer: 7.5% vs. 6.4%, p = 0.01). After adjustment for factors listed above, racial differences were nonsignificant for CVD- (HR = 1.10, p = 0.09) and second cancer-related mortality (HR = 0.98, p = 0.78). Conclusions: The significantly higher late mortality experienced by NHBs is partially explained by differences in sociodemographics, pre-cancer comorbidity and receipt of treatment, allowing identification of targetable causes for racial differences in late mortality in early-stage breast cancer patients.