Currently, based on the results of large randomized clinical trials and the conclusions of the Oxford meta-analysis (EBCTCG. Lancet 1998; 352: 930; Peto R. Fifth Main Meeting of the Early Breast Cancer Trialist's Collaborative Group, Oxford, UK, September 2000) and following internationally accepted treatment guidelines (National Institutes of Health Consensus Development Panel. J Natl Cancer Inst 2001; 93(13): 979; Goldhirsch et al. J Clin Oncol 2001; 19: 3817), the vast majority of primary breast cancer patients is offered adjuvant medical therapy. Regarding adjuvant chemotherapy (CT), the two most important and challenging tasks of the medical oncologist today are (1) to identify who needs CT and (2) to select the CT regimen that will work best for the individual patient, while at the same time minimizing the risk of severe or permanent toxicities. The second question will be the focus of this paper.