Despite nearly 20 years of study, the importance of chemotherapy dose intensity in breast cancer remains unclear. Substantial preclinical data suggest a dose-response relationship, and consistent data document that recipients of substandard dosing have inferior outcomes. The use of increased dose-intensive therapies is costly, may require the use of hematopoietic growth factor support, and can result in significant increases in both short- and long-term toxicities. In patients with metastatic disease, increased dose intensity frequently results in increased response rates. However, these increased responses have not translated into consistent improvements in time to progression or overall survival benefit. In the adjuvant setting, increases in the dose intensities of alkylating agents and anthracyclines have failed to support the concept of dose escalation beyond standard doses. Certain subgroups of patients, such as those whose tumors overexpress HER2/neu, may derive a benefit from more dose-intensive therapies. Early results of randomized trials of high-dose chemotherapy in the treatment of metastatic breast cancer and adjuvant therapy for high-risk, early-stage breast cancer, are provocative. However, the often conflicting data do not support the routine use of this modality outside of the study setting.