The aim of this study was to verify the effectiveness of positron emission tomography (PET) in detecting primary sites in carcinoma of unknown primary (CUP) patients. In this study, CUP represented a group of heterogeneous tumors that shared the clinical manifestation of metastatic carcinoma with no obvious primary site at the time of first diagnosis, which included clinical investigations, computed tomography, magnetic resonance imaging and panendoscopy. We reviewed the records of 24 patients with CUP between January 1995 and December 2009. The patients who demonstrated additional tracer uptake sites other than previously known metastatic lesions by PET scan were done direct biopsies for the sites of accumulation. Patients who had a negative PET scan or for whom the primary site could not be identified by direct biopsies underwent examination under anesthesia of the at-risk occult tumor sites. PET scan demonstrated focal accumulation suspicious for primary tumor in 12 (50.0%) of 24 patients: tonsil 5, nasopharynx 3, hypopharynx 1, tongue 1, larynx 1, and maxillary sinus 1. A subsequent biopsy of these sites revealed primary cancer in 9 (37.5%) of 24 patients: tonsil 5, nasopharynx 1, hypopharynx 1, tongue 1, and maxillary sinus 1. In the remaining three patients, no malignant cells were found by the biopsy of the accumulated area: nasopharynx 2, larynx 1. PET scans increase the yield of primary tumor by 37.5%. The sensitivity, specificity for PET scan were 80.8, 76.9%, respectively. PET scanning is useful in detecting primary cancer of CUP patients.