As (18)F-fluorodeoxyglucose (FDG) is taken up by inflammatory lymph nodes, it could be falsely interpreted as metastasis. Therefore, we evaluated the diagnostic ability of positron emission tomography/computed tomography (PET/CT) for lymph node staging of lung cancer when inflammatory lung disease coexisted. Patients with operable non-small-cell lung cancer and FDG-avid lymph nodes were retrospectively classified into two groups; those with inflammatory lung disease (ILD) and those without it (NILD). Receiver operating characteristic (ROC) curve analysis was performed for maximum standardized uptake value (SUVmax), pattern of FDG uptake, maximum Hounsfield unit, and size, and then the areas under the ROC curves (AUCs) were compared between subgroups. There were 124 patients (ILD/NILD = 38/86) and 396 FDG-avid lymph nodes (ILD/NILD = 140/256). The average number of FDG-avid lymph nodes was greater in ILD (3.7 vs. 2.9, p = 0.039), whereas the proportion of metastasis was higher in NILD (25.4% vs. 11.4%, p = 0.002). With all N1-N3 lymph nodes and the NILD group, the AUC values of all four parameters were significantly greater than 0.5 (p < 0.05), and SUVmax was the most valuable parameter for lymph node metastasis. However, in the ILD group, only the AUC value of SUVmax was significantly greater than 0.5. These results were reproduced when analyses were performed with N1-N2 lymph nodes. In conclusion, SUVmax was the most valuable PET/CT parameter for assessment of lymph node metastasis in patients with operable non-small-cell lung cancer. In addition, it was the only valuable parameter when inflammatory lung disease coexisted.