As cancer treatment options broaden, there is a corresponding increase m the need for Improved methods of detecting various stages and forms of the disease. Only through the use of highly sensitive and discriminating dtagnostic tests can a rational and Informed decision be made from among the clinical interventions available to the physician. The case of prostate cancer is an ideal paradigm of this challenging problem. As with many other types of solid tumors, complete cure can frequently be achieved through established methods of treatment if the malignancy is organ-confined. However, accurate staging of the disease when gross metastases are not evident 1s problemattc because the techniques in use have poor sensitivities for micrometastatic disease. Consequently, since the optimal treatment strategy is different for primary and meta-static disease, the patient may not receive the best initial treatment.