Orthopaedic oncologists are increasingly utilizing positron emission tomography (PET) technology in the initial workup and staging of sarcomas and for monitoring treatment response. We evaluated the use of PET with fluorine-18-fluoro-2-deoxy D-glucose (FDG) to detect occult nonpulmonary metastases in patients < age 30 newly diagnosed with either Ewing's or osteosarcoma, and the impact of this information upon therapeutic decision making. We retrospectively reviewed prospectively collected data (1994-2004) on 55 patients age < 30 years old over a 10 year span. PET detected metastases in 12/55 (22%) of these patients, eight of whom (67%) harbored disease outside the lung; however, only 4/55 (7%) were upstaged to Stage IV specifically due to findings determined by PET alone. Three of 17 (18%) Ewing's sarcoma patients, but only one of 38 (3%) osteosarcoma patients, were upstaged by PET alone. The most important alteration in treatment decisions was the substitution of irradiation in lieu of surgery for local control in Ewing's sarcoma patients. Diagnostic study, level II.