Abstract: Objective : To evaluate the diagnostic value of 18 F-FDG PET/CT in differentiating malignant tumors from benign lesions in the lung. Methods : The data from 74 patients with clinically diagnosed and pathologically confirmed pulmonary lesions were reviewed.Patients with obstructive pneumonia were excluded.These 74 patients were followed up for 3-14 days.Whole body 18 F-FDG PET/CT imaging, breath-controlled CT imaging and thin layer scanning with CT (1.25mm in planer thickness) were performed.The diagnostic criteria includ-ed: (1) the SUVmax of pulmonary lesions ≥2.5mm; (2) a significant increase of SUVmax (≥20％～30％); (3) the morphological information provided by thin planer scanning with CT (planer thickness 1.25mm); (4) curves with changing of SUVmax from the center of the lesion to the chest walls. Results : Fifty-five patients were found to have malignant tumors (32 cases of pulmonary carcinoma, 13 cases of adenocarcinoma, 5 cases of squamous cell carcinoma, 3 cases of bronchioloalveolar carcinoma and 2 cases of small cell carcinoma).Nineteen patients were found with benign tumors, including 13 cases of chronic inflammation, 2 cases of gran-uloma, 2 cases of inflammatory pseudotumor and 2 cases of tuberculosis in benign lesions.The final results of 18 F-FDG PET/CT indicated the presence of 57 malignant pulmonary lesions (54 true-positive results and 3 false-positive results) and 17 benign pulmonary lesions (16 true-negative results and 1 false-negative result).The sensitivity, specificity, accuracy, and positive and negative predictive values of 18 F-FDG PET/CT for diag-nosis of pulmonary lesions were 98.2%, 84.2%, 94.6%, 94.7% and 94.1%, respectively. Conclusion : For pul-monary lesions, SUVmax and delayed SUVmax suggest the uptake foci of FDG.Typical morphological signs from thin planer spinal CT are important for differentiating malignant pulmonary tumors from benign ones, thereby significantly reducing the false-positive and false-negative rates.Curves with changes in SUVmax from the center of the malignant lesions to the chest walls also indicate tumor invasion to peripheral pul-monary tissues.