PURPOSE: To evaluate the predictive value of the early response of 18F-flurodeoxyglucose positron emission tomography (FDG PET) during concurrent chemoradiotherapy (CCRT) for locally advanced non-small cell lung cancer (NSCLC). MATERIALS AND METHODS: FDG PET was performed before and during CCRT for 13 NSCLC patients. Maximum standardized uptake value (SUVmax), mean standardized uptake value (SUVmean), metabolic tumor volume (MTV), and total lesion glycolysis (TLG) were measured and the changes were calculated. These early metabolic changes were compared with the standard tumor response by computed tomograms (CT) one month after CCRT. RESULTS: One month after the completion of CCRT, 9 patients had partial response (PR) of tumor and 4 patients had stable disease. The percent changes of SUVmax (%DeltaSUVmax) were larger in responder group than in non-responder group (55.7% +/- 15.6% vs. 23.1% +/- 19.0%, p = 0.01). The percent changes of SUVmean (%DeltaSUVmean) were also larger in responder group than in non-responder group (54.4% +/- 15.9% vs. 22.3% +/- 23.0%, p = 0.01). The percent changes of MTV (%DeltaMTV) or TLG (%DeltaTLG) had no correlation with the tumor response after treatment. All the 7 patients (100%) with %DeltaSUVmax > or = 50% had PR, but only 2 out of 6 patients (33%) with %DeltaSUVmax < 50% had PR after CCRT (p = 0.009). Likewise, all the 6 patients (100%) with %DeltaSUVmean > or = 50% had PR, but only 3 out of 7 patients (43%) with %DeltaSUVmean < 50% had PR after CCRT (p = 0.026). CONCLUSION: The degree of metabolic changes measured by PET-CT during CCRT was predictive for NSCLC tumor response after CCRT.