We used TaqMan PCR to detect quantitative anomalies of tumor markers in both tumor and serum DNA from esophageal cancer patients. We demonstrated the potential of this methodology by detecting erbB-2 amplifications in a plurality of esophageal tumor samples. These amplifications were corroborated by Southern blots. We then showed the potential of this methodology to detect quantitative anomalies of erbB-2 in serum DNA from individuals with a corresponding amplification in the tumor. The capability of TaqMan PCR to detect abnormalities in serum of esophageal cancer patients creates an opportunity to diagnose esophageal cancer and to monitor the outcome of treatment with a blood test.