* Participants will enroll in this study at the time they are starting their adjuvant therapy for breast cancer. Participants will receive chemotherapy with paclitaxel every week for 12 weeks. They will begin to receive trastuzumab at the same time they begin paclitaxel. Once they have completed the 12 weeks of paclitaxel and trastuzumab, they will receive trastuzumab every 3 weeks or weekly for 40 weeks.
* Participants will be followed with routine assessments such as physical exam and vital signs every 3 months for the first year, and then every 6 months for years 2-5. Then we would like to keep track of the participants medical condition by calling them on the telephone once per year.
Inclusion Criteria:
* Histologically confirmed invasive carcinoma of the breast
* Tumors must be less than or equal to 3cm in greatest dimension
* Must have node-negative breast cancer according to teh AJCC 7th edition
* ER/PR determination is required. ER- and PR-assays should be performed by immunohistochemical methods
* HER-2 positive: IHC 3+ or FISH \>2
* Bilateral breast cancers that individually meet eligibility criteria are allowed
* Patients should have tumor tissue available, and a tissue block of sufficient size to make 15 slides must be sent to DFCI for testing
* Less than or equal to 84 days from mastectomy or from axillary dissection or sentinel node biopsy if the patient's most extensive breast surgery was a breast-sparing procedure
* All tumor should be removed by either a modified radical mastectomy or a segmental mastectomy (lumpectomy), with either a sentinel node biopsy or axillary dissection
* 18 years of age or older
* ECOG Performance Status of 0 or 1
* Adequate bone marrow function, hepatic function, and renal function as outlined in protocol
* Left ventricular ejection fraction of greater than or equal to 50%
* Willingness to discontinue any hormonal agent prior to registration and while on study
* Willingness to discontinue sex hormonal therapy, e.g. birth control pills, prior to registration and while on study
* Patients with a history of ipsilateral DCIS are eligible if they were treated with wide-excision alone, without radiation therapy
* Patients undergoing breast conservation therapy must not have any contraindications to radiation therapy
Exclusion Criteria:
* Pregnant or nursing women
* Locally advanced tumors at diagnosis, including tumors fixed to the chest wall, peau d'orange, skin ulcerations/nodules, or clinical inflammatory changes
* History of prior chemotherapy in past 5 years
* History of prior trastuzumab therapy
* Active, unresolved infection
* Prior history of any other malignancy in the past 5 years, except for early stage tumors of the skin or cervix treated with curative intent
* Sensitivity to benzyl alcohol
* Grade 2 or greater neuropathy per NCI's CTCAv3.0. (Exception: Any chronic neurologic disorder will be looked at on a case-by-case basis by the study chair).
* Active cardiac disease as outlined in protocol.
