Our aim in this study was to evaluate the efficiency of combined positron emission tomography and computed tomography (PET/CT), in the detection of the primary tumor origin in patients with metastatic osseous lesions of cancer of unknown primary origin (CUP). PET/CT was performed for 50 patients with CUP. These patients presented with metastatic osseous lesions that were pathologically proven, radiologically, and/or clinically suspected osseous metastatic lesions with no detected primary site. The number of patients with true positive primary tumor sites was 42 (84 %), with no false positive results, 5 patients with true negative results (10%) and 3 patients with false negative results (6 %). The sensitivity was 93.3 %, specificity was 100 %. Our positive predictive value in this study was 100 % and our negative predictive value for this study was 62.5 %. This study showed an advantage of PET/CT in the detection of the site of the primary tumor in patients diagnosed with CUP.