Chemotherapy regimens for early stage breast cancer have been 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 clinical practice. We extracted individual-level data on chemotherapy administration from the electronic medical records of Kaiser Permanente Northern California (KPNC), a pre-paid integrated healthcare system serving 29 % of the local population. We linked data to the California Cancer Registry, incorporating socio-demographic and tumor factors, and performed multivariable logistic regression analyses on the receipt of specific chemotherapy regimens. We identified 6,004 women diagnosed with Stage I-III breast cancer at KPNC during 2004-2007; 2,669 (44.5 %) received at least one chemotherapy infusion at KPNC within 12 months of diagnosis. Factors associated with receiving chemotherapy included <50 years of age [odds ratio (OR) 2.27, 95 % confidence interval (CI) 1.81-2.86], tumor >2 cm (OR 2.14, 95 % CI 1.75-2.61), involved lymph nodes (OR 11.3, 95 % CI 9.29-13.6), hormone receptor-negative (OR 6.94, 95 % CI 4.89-9.86), Her2/neu-positive (OR 2.71, 95 % CI 2.10-3.51), or high grade (OR 3.53, 95 % CI 2.77-4.49) tumors; comorbidities associated inversely with chemotherapy use [heart disease for anthracyclines (OR 0.24, 95 % CI 0.14-0.41), neuropathy for taxanes (OR 0.45, 95 % CI 0.22-0.89)]. Relative to high-socioeconomic status (SES) non-Hispanic Whites, we observed less anthracycline and taxane use by SES non-Hispanic Whites (OR 0.63, 95 % CI 0.49-0.82) and American Indians (OR 0.23, 95 % CI 0.06-0.93), and more anthracycline use by high-SES Asians/Pacific Islanders (OR 1.72, 95 % CI 1.02-2.90). In this equal-access healthcare system, chemotherapy use followed practice guidelines, but varied by race and socio-demographic factors. These findings may inform efforts to optimize quality in breast cancer care.