Black women are more likely to be diagnosed with advanced stage and other less favorable breast cancer prognostic factors than white women. The aim of this study was to examine the extent to which markers of socioeconomic position accounts for black-white differences in these factors. Our study included 193,969 women diagnosed with invasive breast cancers during 2004-2005 from the National Cancer Database, which represents about 72% of all patients with cancer treated in the United States. Compared to white women, black women are more likely to be diagnosed with breast tumors that are less differentiated (odds ratio (OR) = 2.55, 95% confidence interval (CI) 2.44-2.66), hormone receptor negative (OR = 2.29, 95% CI 2.22-2.37), large (OR = 1.87, 95% CI 1.80-1.95), metastatic (OR = 1.89, 95% CI 1.78-2.00), and lymph node-positive (OR = 1.44, 95% CI 1.40-1.48). In multivariable analyses, adjustment for insurance and area-level educational attainment explained 31-39% of the differences in tumor size and metastasis, but only about 14% of the differences in grade and hormone receptors. After accounting for race and other covariates, uninsured women remained 3.66 (95% CI 3.30-4.07) times more likely to have metastasis and 2.37 (95% CI 2.17-2.58) times more likely to have large tumors compared to privately insured women. Similarly, the risk of having breast cancer with less favorable prognostic factors increased as area-level educational attainment decreased. Extending health insurance coverage to all women is likely to have an effect on reducing racial disparities in the development of breast cancers with poor prognostic factors.