Steroid hormones play a major role in the development and, growth of endometrial and breast cancer. Besides regulating the proliferation of cells susceptible to carcinogens or initiated by carcinogens, steroid hormones can modulate the growth factor system either directly or indirectly in normal and neoplastic endometrial and breast tissue. The present article analyzes the experimental data on the interactions between steroid hormones and growth factor system in these tissues. Epidemiologic studies on the risk of endometrial and breast cancer in oral contraceptive or hormone replacement therapy users are also reviewed.