BACKGROUND: Several factors may contribute to poorer prognosis for obese breast cancer patients, including unfavorable disease features, the influence of fat on estrogen availability, co-morbidity, and socio-demographic factors. Both obesity and estrogen receptor negative (ER-) tumors are more prevalent in black women than in whites in North America. We evaluated obesity and race in relation to outcomes in women with ER-breast cancer. METHODS: Among 4,077 women from National Surgical Adjuvant Breast and Bowel Project clinical trials for node-negative, ER-breast cancer, we evaluated disease-free survival (DFS) and its constituents (tumor recurrence, contralateral breast cancer (CBC), second primary cancers, deaths prior to these events) and mortality in relation to body mass index (BMI) and race, using statistical modeling to account for other prognostic factors. RESULTS: Compared to those of normal weight (BMI or =24.9), DFS hazard was greater for obese (BMI or = 30) women [hazard ratio (HR)=1.16, 95% confidence interval (CI)=1.01-1.33]. Obesity did not increase recurrence hazard, but did influence CBC (HR=2.08, 95% CI=1.22-3.55 in postmenopausal women) and second cancers (HR=1.49, 95% CI=1.06-2.10). Mortality increased with obesity; when partitioned by likely cause, those with BMI or = 35.0 had greater risk of non-breast cancer mortality (HR=1.86, 95% CI=1.21-2.84). Relative to whites and adjusted for BMI, black women had greater hazard for DFS (HR=1.17, 95% CI=1.00-1.38), CBC (HR=1.37, 95% CI=0.94-1.99), and non-breast cancer deaths (HR=2.10, 95% CI=1.45-3.03); risk for deaths likely due to breast cancer was closer to that in whites (HR=1.18; 95% CI=0.93-1.50). CONCLUSIONS: For women with node-negative, ER-breast cancer from clinical trials, obesity did not increase recurrence risk, but was associated with greater risk for second cancers, CBC, and mortality, particularly non-breast cancer deaths. Less favorable prognosis for black women persists in clinical trials, and is in part attributable to non-breast cancer outcomes.