Endocrine therapy is an important modality in the treatment of breast cancer. However, the precise molecular mechanisms underlying the growth inhibitory effect of endocrine therapy are unknown. Recently, it has been shown that breast cancer cells express and secrete polypeptide growth factors that can regulate the growth of the cells through autocrine and/or paracrine pathways. These growth factors are thought to be involved in the response to endocrine therapy. Three different mechanisms have been suggested: (1) stimulation of growth inhibitory peptides; (2) repression of mitogenic peptides; and (3) stimulation of mitogenic growth factors in cells overexpressing the corresponding receptor. This article reviews the scientific evidence on which these hypotheses are based.