Endocrine therapy is important in the treatment of advanced breast cancer. The prototype antiestrogen tamoxifen and the prototype aromatase inhibitor aminoglutethimide have been in clinical use for more than 2 decades, as have synthetic progestin derivatives. Currently, several novel antiestrogens and aromatase inhibitors are used to treat breast cancer. This paper reviews the present knowledge of the clinical pharmacokinetics of these drugs. Drug monitoring in plasma and other body fluids has been improved over recent years by the introduction of sensitive and specific high performance liquid chromatography and gas chromatography-mass spectrometry methods. However, we still lack information on such basic pharmacokinetic parameters as the bioavailability of several of these drugs. It is important to study not only plasma but also tissue drug concentrations.