To assess whether differentiation is possible between residual disease and complete response, 18F-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography was performed in 17 cases of primary squamous cell carcinoma of the thoracic esophagus before (five control cases) or immediately after full-dose irradiation (12 treated cases). The FDG counts in the esophageal tumors were corrected by the dose of injected isotope and the weight of the patient, to produce a standardized uptake value (SUV). Endoscopy for post-treatment evaluation showed almost complete clearance of the tumor in seven treated cases (good-responders) and apparent residual tumor in the other five cases (poor-responders). There was a significant difference in the average median SUV between the controls and the treated (8.1 +/- 1.6 vs. 3.3 +/- 1.1), and between the poor-responders and good-responders (4.1 +/- 0.8 vs. 2.7 +/- 0.9). Three of the good-responders showed locoregional recurrence after four, six, and 18 months. Among the good-responders, the SUV of the cases of local recurrence was not inevitably higher than that of the others. The rate of FDG uptake in irradiated esophageal cancer approximately represents tumor response to radiotherapy, but it seemed impossible to differentiate residual disease from complete response.