The purpose of this study was to estimate the prevalence of women receiving treatment or active surveillance for stage I-III breast cancer in the United States from 2009 to 2012, stratified by patient age and tumor characteristics. In each study year, electronic medical records were used to identify women aged ≥18 years with stage I-III breast cancer and treated or under active surveillance (≥4 visits) at an oncology clinic that contributes data to the Oncology Services Comprehensive Electronic Records database. Prevalence was projected to the national level overall and within strata (by tumor characteristics, year of breast cancer diagnosis, and age). We identified 5,219 female breast cancer patients (18 % <age 50; 58 % <age 65) representing 787,082 (95 % CI 778,012-796,153) women in the US in 2012. At diagnosis, 44 % had stage I, 42 % stage II, and 14 % stage III disease; 69 % had estrogen receptor (ER) and progesterone receptor (PR) expression-positive tumors; 19 % were ER- and PR-negative; and 14 % had triple-negative phenotype (ER-, PR-, and HER2-negative). Overall 56 % of patients received treatment in 2012, 22 % chemotherapy, 8 % biologic therapy, and 36 % endocrine therapy. Treatment prevalence was higher among younger patients and at more advanced disease stages. Approximately half of women with ER-negative, PR-positive, HER2-negative, or triple-negative tumors received chemotherapy. As a conclusion, in 2012, approximately 800,000 women in the US were under treatment or active surveillance for early-stage breast cancer. Treatment prevalence differed by patient age, disease stage, and tumor histology.