Background: We investigated differences in breast cancer mortality between younger (younger than 40 years of age) and older (40 years of age and older) women by stage at diagnosis to identify patient and tumor characteristics accounting for disparities. Study Design: We conducted a retrospective study of women diagnosed with breast cancer in the 1988 to 2003 Surveillance, Epidemiology, and End Results Program data. Multivariate Cox regression models calculated adjusted hazard ratios (aHR) and 95% confidence intervals to compare overall and stage-specific breast cancer mortality in women younger than 40 years old and women 40 years and older, controlling for potential confounding variables identified in univariate tests. Results: Of 243,012 breast cancer patients, 6.4% were younger than 40 years old, and 93.6% were 40 years of age or older. Compared with older women, younger women were more likely to be African American, single, diagnosed at later stages, and treated by mastectomy. Younger women had tumors that were more likely to be higher grade, larger size, estrogen receptor/progesterone receptor-negative, and lymph-node positive (p < 0.001). Younger women were more likely to die from breast cancer compared with older women (crude HR = 1.39; CI, 1.34 to 1.45). Controlling for confounders, younger women were more likely to die compared with older women if diagnosed with stage I (aHR = 1.44; CI, 1.27 to 1.64) or stage II (aHR = 1.09; CI, 1.03 to 1.15) disease and less likely to die if diagnosed with stage IV disease (aHR = 0.85; CI, 0.76 to 0.95). Conclusions: Higher breast cancer mortality in younger women was attributed to poorer outcomes with early-stage disease. Additional studies should focus on specific tumor biology contributing to the increased mortality of younger women with early-stage breast cancer.