The development of cancer is a slow process and it very often takes a number of years before a tumor becomes clinically evident. No current te- nique is sensitive and specific enough to detect tumors at their earliest stage, i. e. , when the tumor is smaller than one billion cells. Consequently, the d- ease is usually diagnosed in an advanced state, very often when it is already beyond the reach of therapeutic strategies. This is the main stumbling block to the secondary prevention that would reduce cancer mortality. There is hope, however, because in the last decade we have witnessed an explosion of reports dealing with tumor markers. In many instances, simple, noninvasive diagnostic tests are becoming available to detect the early signs ofneoplasia. Interest in early detection of neoplasia is growing among those basic scientists, cli- cians, and health professionals who realize that progress in reducing cancer mortality is dependent to a great degree on its early detection and prevention. Some disappointment usually accompanies these discoveries, because what at first glance proves to be promising, is in many instances, applicable only to a limited number of cases. What is the cause of these failures? The answer may be that cancer is a multitude of neoplastic diseases in which endogenous and exogenous etiologic factors contribute either sim- taneously or over a lifetime to the development of disease.