With the approval of pegfilgrastim, the use of dose-dense epirubicin and cyclophosphamide (EC) for breast cancer has become acceptable in Japan. Thus, we aimed to evaluate its safety and tolerability in Japanese patients. Nine breast cancer patients with a high risk of preoperative or postoperative recurrence received EC therapy(epirubicin 90 mg/m(2) and cyclo- phosphamide 600 mg/m(2))for 4 cycles every 2 weeks in combination with a subcutaneous injection of pegfilgrastim (3.6 mg) on day 2 of each cycle. Treatment was discontinued in 1 and extended in 1 of the 9 patients, and the mean relative dose intensity(RDI)was good at 0.93. No serious adverse events were observed, indicating good tolerability. The regimen has potential for use in cases in which the treatment dose needs to be increased. grade 4 neutropenia was observed in all the 9 patients on day 8, with 6 patients developing febrile neutropenia. In Japan, data on changes in neutrophil count associated with pegfilgrastim administration under anthracycline-based chemotherapy are currently insufficient, and further study is required.