Objective: We aimed to investigate the relationship between the use of fluoro-2-deoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT), maximized standardized uptake value(SUVmax) values of tumors, and tumor differentiation and tumor markers during the initial staging of patients with gastric cancer.Materials and Methods: The study included 50 patients (14 women and 36 men; mean age: 63±11 years;age range: 31-80 years) who had undergone initial staging with FDG-PET/CT after the diagnosis of gastriccancer with endoscopic biopsy between January and June 2013. Serum alpha fetoprotein (AFP), carcinogenicantigen 19-9 (CA 19.9), carcinoembryonic antigen (CEA), and C-reactive protein (CRP) levels were measured in patients prior to imaging. PET/CT images were evaluated for primary tumors, locoregional spread,and distant organ metastases, and classified by tumor-node-metastasis staging. Semiquantitative data werecollected by SUVmax measurements in pathological regions of involvement. Data were analyzed statistically.Results: FDG-PET/CT showed primary gastric cancer with a sensitivity of 87%. Imaging findings were normalin 3 patients (1/3; mucinous adenocarcinoma, 2/3; signet-ring cell adenocarcinoma). With FDG-PET/CT, 3/50patients were classified into Stage 1B, 3/50 patients into Stage 2, 5/50 patients into Stage 3A, 5/50 patientsinto Stage 3B, 5/50 patients into 3C and 29/50 patients into Stage 4. The mean SUVmax was calculatedas 11.35±4.3 (poorly differentiated adenocarcinoma: 5.4±1.7; moderately differentiated adenocarcinoma:10.3±4.8) for the primary tumor and 14.9±6.3 for tumor metastasis. A positive correlation was evidentbetween the measured SUVmax and stage and the grade of primary tumor (p<0.05). While the relationshipbetween SUVmax and levels of serum AFP and CRP was statistically significant (p<0.05), the relationshipbetween SUVmax and levels of serum CA 19-9 and CEA was not statistically significant (p>0.05).Conclusion: The SUVmax of primary tumors was associated with the degree of differentiation of primarytumors and the biochemical tumor markers CRP and AFP. The fact that SUVmax of primary tumors is highsupplies clues about the presence of the factors affecting prognosis of the disease.