Recently, much interest has been focused on the diagnostic application and biology of tumor-derived DNA in the plasma and serum of cancer patients. Such interest has resulted in the demonstration of Epstein-Barr virus (EBV) DNA in the plasma/serum of patients with nasopharyngeal carcinoma (NPC). Using quantitative real-time polymerase chain reaction (PCR) technology, circulating EBV DNA has been found not only to correlate with disease staging, but also to provide additional prognostic information. Following treatment, circulating EBV DNA analysis has been shown to provide useful information for the monitoring for tumor recurrence. In addition to NPC, it is envisaged that circulating DNA technology will find wide applications in the detection and monitoring of many other types of malignancies.