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Recently, randomized trials revealed that trastuzumab as adjuvant treatment was effective in human epidermal growth factor receptor 2 (HER2)-positive breast cancer patients. Safety information on adjuvant trastuzumab use in Japanese patients, especially cardiac toxicity data, is needed. We retrospectively reviewed 48 patients with early-stage HER2-positive breast cancer who were treated with curative surgery and adjuvant trastuzumab at the National Cancer Center Hospital East (Kashiwa, Japan). The cardiac safety as well as the short-term efficacy of trastuzumab were evaluated. The median age of the patients was 54 years. All patients received adjuvant or neoadjuvant cytotoxic chemotherapy. Twenty-seven patients (56%) received adjuvant radiation therapy. Forty-four patients (92%) received trastuzumab without concurrent cytotoxic chemotherapy and 4 patients (8%) on taxanes received trastuzumab concurrently. Twenty-five patients completed 1 year of trastuzumab treatment and 5 patients completed 2 years of trastuzumab treatment. Nine patients discontinued trastuzumab treatment, because of progressive disease (1 patient), decrease in left ventricular ejection fraction (LVEF; 2 patients), patient’s refusal (4 patients), and other reasons (2 patients). There were five cardiac events. A decrease in LVEF to less than 50% was seen in 2 patients. The relationship between trastuzumab treatment and the cardiac events was unclear in 3 patients. The median follow-up time was 21.2 months. The disease-free survival (DFS) was 97.5% at 1 year and 92.9% at 2 years. The incidence of cardiac events caused by trastuzumab treatment was low in our analysis. Adjuvant trastuzumab treatment for up to at least 1 year should be safe for Japanese breast cancer patients. [ABSTRACT FROM AUTHOR]