To assess social disparities in breast cancer diagnosis and treatment by comparing the stage at diagnosis and treatment of women diagnosed with breast cancer through a National Breast and Cervical Cancer Early Detection Program (NBCCEDP) for low income and uninsured women in Massachusetts, the Women's Health Network (WHN), to other breast cancer patients in the state. We linked data from the WHN and the Massachusetts Cancer Registry (MCR). We compared 331 WHN women and 13,372 other breast cancer patients in Massachusetts diagnosed from 1995 to 1999. We used logistic regression, controlling for age, race/ethnicity, marital status, region of residence, and stage, where appropriate. Compared to other breast cancer patients reported to the MCR, WHN women were just as likely to be diagnosed at an advanced stage (III or IV), treated with surgery, chemotherapy, and hormone therapy, and treated with complete mastectomy versus partial mastectomy. WHN women were less likely to receive radiation therapy (odds ratio = 0.7; 95% confidence interval = 0.6-0.9), particularly after partial mastectomy, and had a slightly longer time from diagnosis to treatment than other breast cancer patients (p < 0.01). Women diagnosed with breast cancer through a NBCCEDP in Massachusetts had similar stage and treatment patterns as other breast cancer patients in the state, except for the use of radiation therapy.