Positron emission tomography (PET) with fluorine-18 fluorodeoxyglucose (FDG) was performed in 19 patients with brain metastases from non-central nervous system (CNS) neoplasms and one patient with a primary CNS lymphoma. Various histopathologic types were represented by the primary neoplasms in the patients with metastases. Only 21 of the 31 lesions (68%) were detected with FDG PET as discrete, metabolically active foci (relative to surrounding structures). Six of the nondetected lesions may have been nondiscernible owing to their small size and/or isointensity relative to closely apposed normal gray matter. However, four lesions of at least 1.2 cm in diameter showed frankly decreased FDG accumulation relative to normal brain. These findings suggest that studies of FDG accumulation by a variety of non-CNS neoplasms and their CNS metastases are in order and that extrapolation of the successes of FDG PET in imaging of primary glial tumors to imaging of brain metastases should proceed with caution.