Recently, the thymidine analog 3′-deoxy-3′-18F-fluorothymidine (FLT) was suggested for imaging tumoral proliferation. In this prospective study, we examined whether ¹⁸F-FLT better determines proliferative activity in newly diagnosed lung nodules than does ¹⁸F-FDG. Methods: Twenty-six patients with pulmonary nodules on chest CT were examined with PET and the tracers ¹⁸F-FDG and ¹⁸F-FLT. Tumoral uptake was determined by calculation of standardized uptake value (SUV). Within 2 wk, patients underwent resective surgery or had core biopsy. Proliferative activity was estimated by counting nuclei stained with the Ki-67-specific monoclonal antibody MIB-1 per total number of nuclei in representative tissue specimens. The correlation between the percentage of proliferating cells and the SUVs for ¹⁸F-FLT and ¹⁸F-FDG was determined using linear regression analysis. Results: Eighteen patients had malignant tumors (13 with non-small cell lung cancer [NSCLC], 1 with small cell lung cancer, and 4 with pulmonary metastases from extrapulmonary tumors); 8 had benign lesions. In all visible lesions, mean ¹⁸F-FDG uptake was 4.1 (median, 4.4; SD, 3.0; range, 1.0-10.6), and mean ¹⁸F-FLT uptake was 1.8 (median, 1.2; SD, 2.0; range, 0.8-6.4). Statistical analysis revealed a significantly higher uptake of ¹⁸F-FDG than of ¹⁸F-FLT (Mann-Whitney U test, P < 0.05). ¹⁸F-FLT SUV correlated better with proliferation index (P < 0.0001; r = 0.92) than did ¹⁸F-FDG SUV (P < 0.001; r = 0.59). With the exception of 1 carcinoma in situ, all malignant tumors showed increased ¹⁸F-FDG PET uptake. ¹⁸F-FLT PET was false-negative in the carcinoma in situ, in another NSCLC with a low proliferation index, and in a patient with lung metastases from colorectal cancer. Increased ¹⁸F-FLT uptake was related exclusively to malignant tumors. By contrast, ¹⁸F-FDG PET was false-positive in 4 of 8 patients with benign lesions. Conclusion: ¹⁸F-FLT uptake correlates better with proliferation of lung tumors than does uptake of ¹⁸F-FDG and might be more useful as a selective biomarker for tumor proliferation.