PURPOSE: Patients with primary breast cancer who have extensive axillary lymph node involvement have a poor prognosis after conventional adjuvant therapy. We compared intense dose-dense (IDD) adjuvant chemotherapy with conventionally scheduled adjuvant chemotherapy in patients with high-risk primary breast cancer. PATIENTS AND METHODS: In this randomized, phase III trial, a total of 1,284 eligible patients with four or more involved axillary lymph nodes were randomly assigned to receive IDD sequential epirubicin, paclitaxel, and cyclophosphamide (IDD-ETC) every 2 weeks or conventionally scheduled epirubicin/cyclophosphamide followed by paclitaxel every three weeks. The primary end point was event-free survival (EFS). RESULTS: At a median follow-up of 62 months, 5-year event-free survival rates were 62% in the conventional arm and 70% in the IDD-ETC arm, representing a 28% reduction of the relative risk of relapse (P < .001). This benefit was independent of menopausal, hormone receptor, or human epidermal growth factor receptor 2 status. The 5-year overall survival rates were 77% versus 82%, representing a 24% reduction of the relative risk of death (P = .0285). IDD therapy was associated with significantly more nonhematologic and hematologic toxicities, but no treatment-related death occurred. Four occurrences of acute myeloid leukemia or myelodysplastic syndrome (MDS) were observed in the IDD-ETC arm. No severe congestive heart failure was reported. CONCLUSION: IDD-ETC was less well tolerated compared with conventional chemotherapy but significantly improved event-free and overall survivals in patients with high-risk primary breast cancer who had four or more positive axillary lymph nodes.