We evaluated the toxicity of 4 cycles of doxorubicin (60 mg/m2) and cyclophosphamide (600 mg/m2) every 3 weeks (AC 60/600) in adjuvant chemotherapy for breast cancer. Between 1994 and 2003, 62 patients received 6 cycles of doxorubicin (40 mg/m2) and cyclophosphamide (500 mg/m2) every 3 weeks (AC 40/500), and 106 patients received AC 60/600 as adjuvant chemotherapy for breast cancer. The performance status of all patients was 0 or 1. Toxicity was determined using the National Cancer Institute Common Toxicity Criteria (NCI-CTC) ver. 2. Grade 3/4 neutropenia was frequent in AC 60/600 (6.5% vs 24.3%, p < 0.001). However, febrile neutropenia was not significant in either group (1.6% vs 3.8%, p = 0.39). There was also no statistical difference in the toxicity greater than grade 3 of anemia, nausea, vomiting, fatigue, diarrhea and cardiotoxicity. There was no treatment-related death in both groups. The number of patients who completed chemotherapy was higher in those receiving AC 60/600 than in those receiving AC 40/500 (91.9% vs 99.1%, p = 0.026). AC 60/600 is tolerated and feasible in adjuvant chemotherapy of breast cancer in Japanese patients from the viewpoint of toxicities.