Background/aim: This study aimed to retrospectively evaluate the role of 18F-FDG PET/CT imaging in the detection of unknown primary tumor sites in patients with a suspicious malignancy. Materials and methods: We retrospectively examined the 18F-FDG PET/CT images of 50 unknown primary malignancy patients. The malignancy of the lesions with increased 18F-FDG uptake on PET images was defined by interpreting the nondiagnostic CT images that were obtained with the PET study. The primary tumor site was decided according to the combined PET/CT findings, and the results were subsequently confirmed with a histopathological examination. Results: Fifty patients (29 M; 21 F) aged 18-85 years were included in the study. The sample included 32 malignant and 18 benign lesions according to the histopathological evaluation. 18F-FDG PET/CT study accurately identified malignant lesions in 28 (average SUVmax ± SD: 8.27 ± 7.22) and benign lesions in 12 (average SUVmax ± SD: 3.63 ± 3.07) patients; these findings were histopathologically confirmed. PET/CT correctly detected the primary tumor site in 16 (50%) of 32 patients. Conclusion: 18F-FDG-PET/CT identified the primary tumor site well in 50% of our cases. We propose that 18F-FDG PET/CT imaging may help to accurately detect malignant lesions in patients with unknown primary tumors.