The potential of anatomic imaging to improve the quantitative accuracy of functional brain imaging through refined regional definition is widely accepted. However, there are little data addressing the impact of approach to regional localization on quantitation of metabolic images in the absence of gross structural pathology. We compared MRI-based versus PET-based approaches to the analysis of PET 18F-fluorodeoxyglucose (FDG) images using a standard adjustable template based on simple geometric regions. For the MRI-based approach, templates and individual regions were adjusted to each individual's anatomy, whereas the PET-based definition involved only global proportional adjustment of the standard templates. Metabolic rates for glucose and volume-to-whole brain ratios were determined by two operators for 78 volumes of interest in five subjects. Pairwise correlations indicated high interoperator agreement for each approach and high intraoperator agreement for MRI-based versus PET-based metabolic values. The stability of the metabolic rates and ratios among operators and analysis approaches was supported by low coefficients of variation across measurements and small average differences in paired comparisons. Thus, within the current spatial resolution of PET imaging, quantitation of metabolic images is relatively robust to image analysis approach in the absence of gross structural abnormality. To take advantage of the greater quantitative accuracy promised by high-resolution anatomic and functional imaging, more refined delineation of anatomic images will be necessary.