For the integration of new cell biological prognostic factors in daily clinical practice, we need to know not only their prognostic power with respect to prediction of relapse free and overall survival, but also their possible relation to response to endocrine therapy or chemotherapy in order to select adequate treatment for each patient. A large number of cell biological parameters are currently available to predict the prognosis of patients with breast cancer, but it is still difficult to predict the response to treatment accurately. A valuable prognostic factor can be a worthless predictive factor for endocrine therapy or chemotherapy, and vice versa. High tumour levels of ER, PGR, AR and PS2 protein predict a relatively good response to endocrine therapy, whereas EGFR positivity, HER2/neu positivity, aneuploidy, high proliferation indices and possibly high u-PA levels indicate a good chance of a poor response to endocrine therapy in metastatic breast cancer. With respect to chemotherapy, a high proliferation rate and HER2/neu amplification predict a good response to therapy in metastatic disease, whereas MDR gene expression and possibly c-myc amplification are related to a worse response. In conclusion, the newer cell biological parameters can be used to select high and low risk patients and type of systemic treatment and can be used as targets for new treatment modalities.