BACKGROUND: The use of high-dose adjuvant chemotherapy for high-risk primary breast cancer is controversial. We studied its efficacy in patients with 4 to 9 or 10 or more tumor-positive axillary lymph nodes. METHODS: Patients younger than 56 years of age who had undergone surgery for breast cancer and who had no distant metastases were eligible if they had at least four tumor-positive axillary lymph nodes. Patients in the conventional-dose group received fluorouracil, epirubicin, and cyclophosphamide (FEC) every three weeks for five courses, followed by radiotherapy and tamoxifen. The high-dose treatment was identical, except that highdose chemotherapy (6 g of cyclophosphamide per square meter of body-surface area, 480 mg ofthiotepa per square meter, and 1600 mg ofcarboplatin per square meter) with autologous peripheral-blood hematopoietic progenitor-cell transplantation replaced the fifth course of FEC. RESULTS: Of the 885 patients, 442 were assigned to the high-dose group and 443 to the conventional-dose group. After a median follow-up of 57 months, the actuarial 5-year relapsefree survival rates were 59 percent in the conventional-dose group and 65 percent in the high-dose group (hazard ratio for relapse in the high-dose group, 0.83; 95 percent confidence interval, 0.66 to 1.03; P=0.09). In the group with 10 or more positive nodes, the relapse-free survival rates were 51 percent in the conventional-dose group and 61 percent in the high-dose group (P=0.05 by the log-rank test; hazard ratio for relapse, 0.71; 95 percent confidence interval, 0.50 to 1.00). CONCLUSIONS: High-dose alkylating therapy improves relapse-free survival among patients with stage II or III breast cancer and 10 or more positive axillary lymph nodes. This benefit may be confined to patients with HEK-2/neu-negative tumors.