Some endometrial cancers and endometrial adenocarcinoma cell lines show amplified expression of proto-oncogenes (fos, fms, myc, myb, neu, and erb-B) and augmented production of growth factors (colony-stimulating factor 1, epidermal growth factor, transforming growth factor-alpha, and transforming growth factor beta) and epidermal growth factor receptor. Oncogene expression, the presence of estrogen and progesterone receptors, and the fraction of cells in S phase are useful biochemical prognostic indicators of clinical outcome, and markers recognized by monoclonal antibodies are available for use in following the clinical course of the disease and responses to treatment. In vivo and in vitro studies on normal and neoplastic tissues are providing evidence of paracrine influences on epithelial cell proliferation. Long-term administration of tamoxifen as adjuvant therapy for breast cancer has recently been found to increase the risk for development of endometrial cancer.