Fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET) has been used successfully for the staging and re-staging of breast cancer. Another significant indication is the evaluation of therapy response. Only limited data are available on the use of FDG-PET in breast cancer after radiation therapy. The same holds true for chemotherapy. Only the therapy response in locally advanced breast cancer after chemotherapy has been investigated thoroughly. Histopathological response could be predicted with an accuracy of 88-91% after the first and second courses of therapy. A quantitative evaluation is, of course, a prerequisite when FDG-PET is used for therapy monitoring. Only a small number of studies have focussed on hormone therapy. In this context, a flare phenomenon with increasing standardised uptake values after initiation of tamoxifen therapy has been observed. More prospective multicentre trials will be needed to make FDG-PET a powerful tool in monitoring chemotherapy in breast cancer.