Purpose: The aim of this study was to evaluate the role of whole body 18F-FDG PET/CT imaging in the detection of primary tumors in patients with a metastatic cancer from an unknown primary site. Methods: The study population consisted of 43 patients with a biopsy proven metastatic disease, negative conventional diagnostic procedures (including CT/MRI/endoscopic procedures) and a whole body 18F-FDG PET/CT examination. Patients' records were retrospectively analyzed. According to the final pathologic diagnoses, rate of detection of the primary tumor site was determined. Additionally, overall patient survival was calculated to evaluate the prognostic value of 18F-FDG PET/CT findings. Results: A primary tumor site was shown by 18F-FDG PET/CT in 24 patients (24/43; 55.8%). In 18 patients 18F-FDG PET/CT scans were negative (18/43; 41.8%). In a patient with an adenocarcinoma metastasis 18F-FDG PET/CT was falsely positive for an inflammatory lesion in the lung. Among the 18F-FDG PET/CT positive and negative groups median overall survival was not significantly different (log-rank p=0.573). Conclusion: Whole body 18F-FDG PET/CT imaging has a high rate of detection of a primary tumor in patients with a carcinoma of unknown origin.