The use of human tumor xenografts grown in immunodeficient animals as a model for human cancers is well established and their value depends on the extent to which their characteristics reflect the properties of a particular cancer in the clinical situation. For endocrine-sensitive tumors, the retention of hormone receptors in xenografts and their responsiveness to hormonal stimuli are essential criteria if they are to serve as appropriate models. Provided these criteria are met, such experimental systems allow detailed studies of the effects of hormones on hormone-sensitive tumor cells and the potential of endocrine therapies in an in vivo system (1 ).