Objective: 18F-Fluorine fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) imaging is considered the standard imaging modality for patients with non-small cell lung carcinoma. The aim of this study was to compare clinical staging (cTNM) performed with 18F-FDG PET/CT and surgical staging (sTNM) in patients with non-small cell carcinoma treated with surgery. Material and methods: We performed a retrospective analysis of 99 surgical patients with non-small cell carcinoma who underwent 18F-FDG PET/CT examination. Semiquantitative measures were calculated from the primary lesions and mediastinal lymph nodes. Findings of cTNM were compared with final surgical–pathological evaluation. Subjects were divided into two groups as postsurgical cTNM changed and cTNM unchanged. Patients in the cTNM changed group were further classified as postsurgical upstaged (US) and downstaged (DS). Results of the US patients were compared with the results of the remaining patients consisting of cTNM unchanged and DS to evaluate the predictable roles of semiquantitative parameters for postsurgical upstaging. To determine mediastinal tumoral involvement, cut-off values were obtained from calculated semiquantitative results of FDG uptakes in lymph nodes. A p value < 0.05 was considered statistically significant. Results: Subjects were aged 40–82 years with a mean age of 64.78 ± 8.70 years. Classification agreement was observed in 43 patients (43%) and in 57%, postsurgical stage migration was seen. Concurrence of cTNM and sTNM was more pronounced in the T1 and N0 subsets which were 84% and 74%, respectively. The lowest concurrence was observed in N1 classification followed by T4 and N2 (1%, 50% and 58%, respectively). Change in T staging occurred in 20 of 56 (36%), in N staging 22 of 56 (39%) and change in T and N in 14 patients (25%). Distribution of US and DS patients in the cTNM changed group was 43% (24 of 56) and 57% (32 of 56), respectively. Results of semiquantitative measures were significantly higher in US patients than the results of the group consisting of DS patients and cTNM unchanged patients, for all parameters. Cut-off value calculated from mediastinal uptakes was most specific for metastases in MTV (metabolic tumor volume) with an acceptable sensitivity (90% and 67%, respectively). Conclusions: The concordance between cTNM and sTNM was better in staging T category compared to N stations. Semiquantitative measures of primary tumor may play a role in predicting postsurgical upstaging. Taking MTV into consideration in the mediastinal region may be more valuable than other parameters in the assessment of nodal involvement.