Objectives: F-18-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) is extensively used to diagnose and stage of lung cancer. The aim of the current study was to investigate the correlation of demographic, clinical, pathological and radiological factors with primer tumor FDG Uptake in patients with lung cancer. Materials and methods: This cross-sectional, clinical study was performed on a total of 57 lung cancer patients newly diagnosed that underwent FDG PET/CT. In addition to descriptive variables, histopathological diagnosis, tumor site and size, hemoglobin level, red cell distribution width, neutrophil to lymphocyte ratio were noted for each patient. The correlation of these variables to SUVmax values in FDG PET/CT was investigated. Results: A total of 57 patients (4 women, 53 men) with an average age of 60.8 +/- 9.4 (range: 33-89) participated in the study. Histopathological diagnoses were consistent with squamous cell carcinoma (28, 49.1%), adenocarcinoma (15, 26.3%) and small cell cancer (14, 24.6%). The SUVmax of primary tumor was positively correlated with tumor size (P<0.001). The tumor SUVmax of squamous cell carcinoma (SqCC) (17.49 +/- 8.37) was higher than that of adenocarcinoma (AC) (12.80 +/- 4.77) and small cell carcinoma (SCC) (12.40 +/- 5.80) (P=0.038). Conclusion: SUVmax value was significantly higher for squamous cell carcinoma and it SUVmax values in PET scans was found to be positively correlated with tumor size. This study suggests that, tumor size and histologic subtype had influences upon FDG uptake in lung cancer.