The purpose of this study was to investigate the potential of using positron emission tomography (PET) with 18F-labeled fluoro-2-deoxy-D-glucose (FDG) to detect unknown primary tumors of cervical metastases. Thirteen patients with various histologic types of cervical metastases of unknown primary origin were studied. Patients received 185-370 MBq FDG intravenously and were scanned from 30 min after injection onward. Whole-body scans were made with a Siemens ECAT 951/31 PET camera. PET identified the primary tumor in four patients: plasmocytoma, squamous cell carcinoma of the oropharynx, squamous cell carcinoma of the larynx, and bronchial carcinoma, respectively. All known metastatic tumor sites were visualized. PET did not identify a primary tumor in one patient in whom a squamous cell carcinoma at the base of the tongue was found in a later phase. In the remaining eight patients, a primary lesion was never found. The follow up ranged from 18 to 30 months. A previously unknown primary tumor can be identified with FDG-PET in approximately 30% of patients with cervical metastases. PET can reveal useful information that results in more appropriate treatment, and it can be of value in guiding endoscopic biopsies for histologic diagnosis.