For patients with estrogen receptor (ER) positive breast cancer, endocrine therapy plays a major role in both the adjuvant and palliative setting. For adequate treatment decision-making it is crucial to obtain up-to-date information on the ER-status of the tumor(s), since ER-expression is the sole predictor for response to endocrine therapy. Moreover, ER-status can change during the course of disease in up to 30% of the patients, and therefore treatments based on the ER-status of the primary tumor may be inadequate. Positron emission tomography (PET) imaging of ER-expression by use of the tracer 16α-[18F]fluoro-17β-oestradiol (FES) can give functional information about the ER-status of all lesions within the body. The aim of this thesis is to address the clinical potential of the FES-PET technique in breast and ovarian cancer patients.