We investigated whether uptake of the thymidine analogue 3-deoxy-3-[(18)F]fluorothymidine ([(18)F]FLT) reflects proliferation in solitary pulmonary nodules (SPNs). Thirty patients with SPNs were prospectively examined with positron emission tomography. Standardized uptake values were calculated for quantification of FLT uptake. Histopathology revealed 22 malignant and 8 benign lesions. Proliferation was evaluated by Ki-67 immunostaining and showed a mean proliferation fraction of 30.9% (range, 1-65%) in malignant SPNs and <5% in benign lesions. Linear regression analysis indicated a significant correlation between FLT-standardized uptake values and proliferative activity (P < 0.0001; r = 0.87). FLT uptake was specific for malignant lesions and may be used for differential diagnosis of SPNs, assessment of proliferation, and estimation of prognosis.