We carried out a study to evaluate the contribution of positron emission tomography with (18)F-fluorodeoxyglucose (PET-FDG) in the diagnosis and therapeutic care of patients presenting with metastases of unknown primary. PET-FDG was prospectively performed in 51 patients. The PET-FDG data were confirmed histologically or by a follow-up on average at 13 months. PET-FDG identified the primary in 24 percent of cases, and detected the presence of additional metastases in 41 percent of cases. PET-FDG led to a therapeutic modification for 12 patients (24 percent). Furthermore, the therapeutic impact seems more marked in localized forms than in the multifocal. This broad exploratory study confirms the important role of PET-FDG in the diagnosis and therapeutic management of patients with metastases of unknown primary.