In the treatment of breast cancer, anthracycline-containing combinations are frequently used as adjuvant therapy or to treat patients with metastatic disease. However, most patients with metastatic disease who are treated with these combinations develop progressive disease and a significant proportion of patients, after receiving anthracycline-containing adjuvant therapy, experience recurrent disease. Patients who develop recurrent disease while receiving adjuvant therapy and those whose metastatic disease progresses without an objective response while on treatment to control the disease, are among those defined as having primary refractory disease. These patients have a poor prognosis. In other patients whose breast cancer is treated with anthracycline-containing combinations, defining the degree of resistance requires careful consideration of the type of response to therapy (complete response, partial response or no change in disease status) the duration of response and, for patients in the adjuvant setting, the length of the disease-free interval.