Background: Chemotherapy regimens for early-stage breast cancer have been extensively tested by randomized clinical trials, and specified by evidence based-practice guidelines. However, little is known about the translation of trial results and guidelines to oncology practice. Methods: We extracted individual-level data on chemotherapy administration from the electronic medical records of Kaiser Permanente Northern California (KPNC), a pre-paid integrated health-care delivery system serving 29% of the local population. We linked data to the California Cancer Registry, incorporating demographic and tumor factors, and performed multivariable logistic regression analyses on the receipt of specific chemotherapy regimens. Results: We identified 6,178 women diagnosed with stage I to III breast cancer at KPNC during 2004 to 2007; 2,735 (44.3%) received at least one chemotherapy infusion at KPNC within 18 months of diagnosis. Factors associated with receiving chemotherapy, and specifically receiving anthracyclines, taxanes, and/or trastuzumab, included young age, large tumor size, involved lymph nodes, hormone receptor-negative or HER2/neu-positive tumors, and high tumor grade; comorbid conditions were inversely associated with chemotherapy use (heart disease for anthracyclines, neuropathy for taxanes). We observed less chemotherapy use by unmarried women, less anthracycline and taxane use by low-socioeconomic status (SES) non-Hispanic whites, and more anthracycline use by high-SES Asian/Pacific Islanders (versus high-SES non-Hispanic whites). Concordance with relevant measures of the American Society of Clinical Oncology Quality Oncology Practice Initiative (QOPI) was highest among younger women with larger, higher grade tumors. Conclusions: In this health care organization with essentially equal access, we discovered that chemotherapy use was concordant with practice guidelines, yet may vary according to socio-demographic factors. These findings may inform efforts to optimize treatment, and guide studies of quality in breast cancer care.