A major problem in the management of patients with cancer is the lack of specific tumor markers for the early detection, the accurate prediction of biological behavior and for accurate assessment of prognosis. A new and exciting answer to this problem may now become available following the discovery of specific genes associated with malignancy. The role of such genes and their products are now being identified and their role in cancer is under intense investigation. On a clinical level, these genes and their products may allow us to improve our understanding of disease etiology and provide more precise diagnostic, prognostic and therapeutic characterization of individual tumors. This paper discusses the possibilities of using the altered expression of oncogenes and tumor suppressor genes and their products in neoplastic tissues as markers for the diagnosis and prognosis of malignant disease. These data support the view that detailed analysis of such gene expression has the potential to predict a tumor's behavior as well as the response to different treatment modalities.