Purpose: We evaluated correlation of ¹⁸F-FDG uptakes, therapeutic response and relapse in pre-treatment ¹⁸F-FDG PET/CT in patients with SCLC. Materials and methods: We included 26 patients with pathologically proven small cell lung cancer. Total 102 lesions (26 lungs, 69 lymph nodes and 8 metastatic lesions) were evaluated. All patients underwent ¹⁸F-FDG PET/CT for staging. The maxSUV was used as a parameter of ¹⁸F-FDG uptake. The patients were divided into responders and non-responders according to response criteria on chest CT scan after 3 cycles of chemotherapy. We compared maxSUV between two groups by using independent t-test. To access correlation with ¹⁸F-FDG uptake and relapse, maxSUV and interval time to relapse was analyzed by correlation analysis. The cutoff value of maxSUV was evaluated by ROC curve. Results: Twelve-one patients (81%) were responders and five patients were non-responders on follow-up chest CT scan. The mean maxSUV of main lung lesions in responders and non-responders were 14.15¡¾3.72 and 9.17¡¾2.15, respectively. The maxSUV in the responders was significantly lower than that in non-responders ( p<0.05). According to ROC curve, point of cut that predicts therapeutic response was 8.98 with 100% sensitivity and 57% specificity. The correlation analysis between ¹⁸F-FDG uptakes and interval time to relapse showed a significant negative correlation ( p<0.05, r=-0.757). Conclusion: The pre-treatment ¹⁸F-FDG uptake of responders was significantly lower than that of non-responders. Patients with high ¹⁸F-FDG uptake in pre-treatment ¹⁸F-FDG PET/CT relapse earlier. (Nucl Med Mol Imaging 2007;41(6):538-545)