Research has shown that Hispanic women in the United States are diagnosed with breast cancer at more advanced stages and initiate treatment later than non-Hispanic white women. We investigated whether stage at breast cancer diagnosis and receipt of primary therapy differ by ethnicity and birthplace among US-born Hispanic, foreign-born Hispanic, and white women. We studied 31,012 Hispanic women and 372,313 white women with a first diagnosis of invasive breast cancer during 1988 and 2005 living in a SEER area. We used multinomial logistic regression to assess the association of ethnicity and birthplace with stage at diagnosis and, among women with stage I or II cancers, primary therapy [mastectomy, breast-conserving surgery (BCS) with radiation, BCS without radiation], adjusting for other patient and tumor characteristics. Rates of stage at diagnosis differed significantly by race/ethnicity and birthplace (P < 0.001). Foreign-born Hispanics had lower adjusted rates of stage I breast cancer at diagnosis (35.4%) than US-born Hispanics (40.6%), birthplace-unknown Hispanics (42.3%), and whites (47.4%). Receipt of primary therapy also differed significantly by race/ethnicity and birthplace (P < 0.001). Foreign-born Hispanics and birthplace-unknown Hispanics had lower rates of BCS with radiation (34.9%, 30.7%) than US-born Hispanics (41.5%) and whites (38.8%). Foreign-born Hispanic women in the United States have a lower probability of being diagnosed at earlier stages of breast cancer and, for women with early-stage disease, of receiving radiation following BCS compared to US-born Hispanics and whites. Identifying factors mediating these disparities may help in developing culturally and linguistically appropriate interventions and improving outcomes.