PURPOSE: To evaluate the effect of obesity on survival among black women and white women with invasive breast cancer and to determine whether obesity explains the poorer survival of black women relative to white women. PATIENTS AND METHODS: We observed 4,538 (1,604 black, 2,934 white) women who were 35 to 64 years of age when diagnosed with incident invasive breast cancer between 1994 and 1998. Multivariate Cox regression models were used to examine the effect of body mass index (BMI, in kilograms per square meter) 5 years before diagnosis on risk of death from any cause and from breast cancer. RESULTS: During a median of 8.6 years of follow-up, 1,053 women died (519 black, 534 white), 828 as a result of breast cancer (412 black, 416 white). Black women were more likely to die than white women (multivariate-adjusted relative risk [RR], 1.33; 95% CI, 1.16 to 1.53). Compared with women with BMI of 20 to 24.9 kg/m(2), those who were obese (BMI ≥ 30 kg/m(2)) had a greater risk of all-cause mortality (RR, 1.23; 95% CI, 1.04 to 1.47) and breast cancer-specific mortality (RR, 1.20; 95% CI, 0.99 to 1.46). These associations were observed among white women (all-cause RR, 1.54; 95% CI, 1.21 to 1.96; breast cancer RR, 1.46; 95% CI, 1.11 to 1.92), but not among black women (all-cause RR, 1.03; 95% CI, 0.81 to 1.29; breast cancer RR, 1.02; 95% CI, 0.79 to 1.33). CONCLUSION: Obesity may play an important role in mortality among white but not black patients with breast cancer. It is unlikely that differences in obesity distributions between black women and white women account for the poorer survival of black women.