Research using experimental models of human breast cancer has broadened our understanding of the possible biochemical pathways regulating breast cancer growth. Breast cancer cells express receptors for and respond to a variety of steroid and polypeptide hormones and growth factors. Specific oncogenes are also expressed in breast cancer cells, and levels of expression may relate to tumor growth and aggressiveness. Recent studies have shown that breast cancer cells can even synthesize and secrete various growth factors that could stimulate tumor growth through autocrine and/or paracrine mechanisms. Secretion of some of these growth factors is regulated by estrogen, providing a possible mechanism for estrogen induced growth. Knowledge of these growth regulatory pathways has potentially important clinical implications. Blockade of these pathways offers new possible treatment strategies, much as antiestrogens have been used to inhibit tumor growth. Quantification of the expression of certain oncogenes, growth factor receptors, or the growth factors themselves, may provide prognostic information for the individual patient. Finally, it is plausible that measurement of these tumor products in body fluids might provide tumor markers that are useful in the diagnosis and treatment of breast cancer.