Chemotherapy is widely used to treat early stage breast cancer (ESBC). Reductions and delays in dose administered--e.g., due to advanced age or febrile neutropenia (FN)--are generally believed to increase risk of disease progression and reduce survival. Little is known about incidence of reduced chemotherapy dose intensity among women with ESBC in the current era of US clinical practice. This study employed a retrospective cohort design and electronic medical records from > 65 community oncology/hematology clinics in > 35 states (2004-2010). The study population comprised adult women who received myelosuppressive chemotherapy for ESBC (stages I-IIIA). For each such woman, each unique cycle of chemotherapy within their first observed course was identified. Incidence of chemotherapy dose delays (≥ 7 days for any drug in ≥ 1 cycles), chemotherapy dose reductions (≥ 15% for any drug in ≥ 1 cycles), and low chemotherapy relative dose intensity (RDI <85% over the course) relative to published reference standards were descriptively analyzed for the seven most-frequently planned regimens in the study database. A total of 2,228 women (70% of the subjects who received chemotherapy for ESBC and met other selection criteria) initiated 1 of the 7 most-frequently planned regimens. Mean age of subjects was 54 years and 69% received primary prophylaxis against FN with a colony-stimulating factor. Incidence of dose delays, dose reductions, and low RDI was 31, 24, and 26%, respectively; low RDI typically was due to premature treatment discontinuation. For patients (n = 626) receiving the most common regimen (dose-dense AC-T: doxorubicin/cyclophosphamide, Q2 × 4 cycles, paclitaxel or docetaxel, Q2 × 4 cycles), incidence of dose delays, dose reductions, and low RDI was 42, 29, and 32%, respectively. In the current era of US clinical practice, chemotherapy dose delays and dose reductions are common among women with ESBC receiving frequently used myelosuppressive dose-dense, as well as conventional, chemotherapy regimens.