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Introduction: F-18 fluorodeoxyglucose (F18-FDG) positron emission tomography-computed tomography (PET/CT) is a valuable functional imaging modality for the clinical diagnosis which provides physiological information based on the altered tissue metabolism. Aim: This study aims to investigate the effectiveness of F-18 fluorodeoxyglucose (F18-FDG) positron emission tomography-computed tomography (PET/CT) in preoperative staging and postoperative local recurrence and distant metastases in patients with rectal cancer. Material and methods: The imaging of 726 patients with rectal cancer who were operated on at Istanbul University, Istanbul School of Medicine and had F18-FDG PET/CT, CT, and magnetic resonance imaging (MRI) scans between September 2005 and October 2016 were retrospectively analyzed. Of these patients, 170 who had pre- and postoperative PET/CT scans, had their CT scans included in the study. The sensitivity and specificity of PET/CT in preoperative staging and detection of postoperative local recurrence and distant metastases were analyzed. Results: Of the patients, 101 were males and 69 were females with a median age of 62.27 (range, 31 to 89) years. The sensitivity and specificity of preoperative PET/CT in detecting liver metastases were 100% (confidence interval [CI]: 66.37–100%) and 94.2% (CI: 89.72–100%), respectively (Cohen’s kappa [κ]: 1.00; P < 0.001). The sensitivity and specificity of postoperative PET/ CT in diagnosing liver metastases were 100% (CI: 88.06–100%) and 98% (CI: 9–100%), respectively (Cohen’s κ: 1.00; P < 0.001). The sensitivity and specificity of preoperative PET/CT in detecting lung metastases were 100% (CI: 66.37–100%) and 91.8% (CI: 89.72–100%), respectively (Cohen’s κ: 1.00; P < 0.001). The sensitivity and specificity of postoperative PET/CT in detecting lung metastases were 100% (CI: 91.4–100%) and 96% (CI: 95.8–100%), respectively (Cohen’s κ: 1.00; P < 0.001). The sensitivity and specificity of PET/CT in preoperative staging of pathological pelvic lymph nodes were 100% (CI: 63.06–100%) and 94.29% (CI: 80.84–99.3%), respectively (Cohen’s κ: 0.860; P < 0.001). The sensitivity and specificity of postoperative PET/CT in detecting local recurrences were 100% (CI: 78.2–100%) and 76.74% (CI: 61.37–88.24%), respectively (Cohen’s κ: 0.219; P < 0.08). Results: Given the fact that PET/CT can detect all primary rectal cancer in preoperative staging, it can be effectively used in selected cases, particularly in those suspected of local and advanced disease and with metastases (T3N0, T3N1, and/or T4N1). Due to a relatively low specificity in detecting local recurrences postoperatively, PET/CT can be combined with further modalities.