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Background: Randomized clinical trials showed the benefit of adjuvant trastuzumab-based chemotherapy (ATBC) for node-positive and/or >1 cm HER2+ breast carcinomas. No efficacy data have been published on ATBC in large series of pT1abN0 HER2+ tumors., Patients and Methods: This retrospective study evaluated 276 cases of pT1abN0 HER2+ breast tumors in eight French cancer centers. Factors associated with prognosis and ATBC prescription were analyzed., Results: A total of 129 cases (47%) were treated with ATBC (ATBC+), 19 with chemotherapy alone, 5 with trastuzumab alone, and 123 (45%) with neither trastuzumab nor chemotherapy (ATBC-). ATBC use was associated with the date of diagnosis (before or after June 2005) and with poor prognostic features. At a median follow-up of 44 months, there were 13 recurrences in the ATBC- group and 2 in the ATBC+ group. ATBC was associated with a significant survival benefit (99% 40-month disease-free survival for ATBC+ versus 93% for ATBC- cases; P = 0.018). Lack of hormone receptors (HRs) and the presence of lymphovascular invasion (LVI) were significantly associated with a poor prognosis and a greater benefit of ATBC., Conclusions: ATBC was associated with a significantly reduced risk of recurrence in pT1abN0 HER2+ tumors, and was more beneficial in HR- and/or LVI+ tumors.