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PURPOSE To investigate the features of 18F-Fluoro-D-glucose positron emission tomography/computed tomography (18F-FDG PET/CT) in differentiating synchronous multiple primary lung cancers (sMPLC) from intropulmonary metastasis (IM). MATERIAL AND METHODS Fifty-nine patients with two synchronous primary lung cancers were selected and 23 lung cancer patients with an additional solitary IM cancer were chosen as the control group between January 2009 and January 2019. Maximum standardized uptake values (SUVmax) on PET/CT was determined for each tumor. The SUVmax ratio between the two tumors was determined and receiver operating characteristic curve analysis was used to evaluate the diagnostic performance. RESULTS The difference of SUVmax ratio between sMPLC (2.3 ± 1.6) and IM (1.5 ± 0.4) was significant, p < 0.01; the area under the curve of the SUVmax ratio was 0.78 with the optimal cutoff value 1.7 (sensitivity 62.7% and specificity 82.6%, p < 0.001). CONCLUSION The SUVmax ratio between two tumors may be helpful in differentiating sMPLC from IM, independent studies with bigger size were needed to further confirm the findings.